It sounds like "rrrrrrrrrrrrrrrrrrm"

October 20, 2010 12:26 PM

Why did my car start roaring whenever I press the accelerator? Should I be concerned?

This past Saturday I had an oil change and replaced my fuel filter. On Sunday morning I noticed a new sound while driving; it could be the result of something Jiffy Lube did, or it could be a coincidence. The weather here started getting Fall chilly about a week ago too (I park outside).

Basically, when I accelerate I hear the engine rev up as normal, but now I'm hearing a second revving-sound too. It's lower in tone and loud enough for me and my wife, who's always a passenger in this car, to notice without listening for it. It's only present when I accelerate; driving at a constant speed sounds normal. I tried revving up while out of gear and didn't hear it then.

Because it's a louder-than-normal engine sound, I suspected my exhaust system and took the car to Meineke for an inspection. They couldn't find anything apart from a slightly wobbly exhaust pipe of some sort under the middle of the vehicle. The mechanic said it couldn't be tightened, but maybe it's expanded/contracted with the weather over the years and now it vibrates when I drive. I'm not comfortable with this "answer." The noise started too suddenly, and is loud enough that I'm concerned about doing damage to my car as I drive. And the noise doesn't sound like a rattle to me.

What could the noise possibly be? The car is a 2003 Subaru Forester with just over 100K miles on it. Should I get a second opinion?

Related question: The Meineke mechanic also found and showed me that the strut on one of my back wheels is in fairly bad shape. Its complement is fine but their policy is to replace struts in pairs only. That would be a bill of about $600. I really want to avoid that kind of expense, but what could the consequences be?

posted by The Winsome Parker Lewis to travel & transportation (15 answers total)

I wonder if Jiffy Lube messed around with your RPM settings. This happened to me once after taking my car in for an oil change and when I took it to a mechanic, he just toned down the RPMs and I was fine.
posted by parakeetdog at 12:36 PM on October 20, 2010

It sounds like an exhaust blow to me - perhaps a manifold crack that only shows itself when the car is accelerating due to the movement of the engine against the mounts at that point. The only way you would get a louder 'engine sound' is through hearing more of the engine working - ie hearing more exhaust.

With a high mileage car, it is possible that your engine mounts could allow a decent amount of movement when under load. This would potentially open up a crack in a mating surface or joint to allow exhaust gasses to escape. It is likely, although unusual, that this crack closes up when the engine is not loaded (ie sits in the middle of the engine mount movement).

What sort of noise is it? If it is smoother and 'boomy' then it will be further back in the exhaust system. The more 'poppy' or 'crackly' and harsh the noise is, the nearer the engine the hole will be.

The only other option would also be engine/gearbox mount related, in that the noise could be the engine noise resonating through the body shell - is it as loud outside the car (ie with the windows open) as it is inside? If it is? It's likely to be an exhaust hole. If it isn't, it is the engine contacting the body under this acceleration load and transmitting the vibration to the body shell.

I wonder if Jiffy Lube messed around with your RPM settings.

Please don't guess in Askme. Factual based answers are much, much more helpful.
posted by Brockles at 12:43 PM on October 20, 2010

I can say the consequences of replacing only one strut is that the other one will snap soon too, and it's cheaper to do them both at once. (My first car lost all four struts in sequence - of course, the mechanic claimed (and charged like) he was replacing them in pairs, but I highly, highly doubt that now.)
posted by restless_nomad at 12:49 PM on October 20, 2010

Are you running your heater now, when you weren't before? Could be your blower motor for the heater, responding to the higher voltage being supplied when you rev the engine.
posted by cosmicbandito at 12:52 PM on October 20, 2010

I've had this sort of noise happen with a few different cars, and it was always in the flex pipe, which is the short section of flexible braided metal hose in the exhaust system, usually right behind the engine.

It's there to let the engine move around without the entire exhaust system moving around with it, and eventually the braided metal starts to fall apart. It was always fairly subtle to start with, and most noticeable with the engine under load, when a lot of exhaust gas is being produced and maybe also when it has flexed into a position where the holes open up a bit.

Before long, though, it disintegrates to the point where it makes noise all the time. Noise like the world is coming to an end. You could just wait and see if it starts getting worse, and if so, fairly soon it will be bad enough that a mechanic or muffler shop won't have any trouble finding the source.
posted by FishBike at 12:52 PM on October 20, 2010

My initial though is also exhaust leak.

If it's noticeable but not all that loud, another possibility might be intake noise. Did Jiffy Lube check or replace the air filter? If so, they may not have put the air intake (a big black plastic snorkel that runs from the front of the engine compartment to the top of the engine) back together quite right, and you're hearing noise from air getting sucked into the engine that normally gets muffled by the intake/air filter.
posted by zombiedance at 12:56 PM on October 20, 2010

One of the common problems on a Subaru are loose exhaust heat shields. They are basically this sheets of metal around and above your exhaust system to deflect heat. They are secured to the body of the vehicle with just a few spot welds. Once one or two spot welds let go, you get that rattling sound. I would bet on this being the problem.
posted by sanka at 12:58 PM on October 20, 2010

My thought was a bad cooling fan clutch. Does it have a fan that is connected to the crank (big pulley) or is it electric?
posted by WhiteWhale at 1:07 PM on October 20, 2010

Good advice, thank you!

FishBike, I believe that's the pipe the Meineke mechanic showed me. I'll keep an eye (or ear) on it and see if I gets worse the way you described.

I don't know a ton about cars but I'll check the air intake zombiedance mentioned. Jiffy Lube did in fact remove my air filter and put it back when they saw it was fine. Because of the timing of this whole thing, I wouldn't be surprised if that was the case. I don't believe Meineke even looked under the hood, so they wouldn't have spotted it. Should be an easy fix if that's the case.
posted by The Winsome Parker Lewis at 1:18 PM on October 20, 2010

My guess is intake noise, too, but I see zombiedance got to it before me. Could just be as simple as them not snapping down the cover on the intake properly and it's sucking in air.
posted by InsanePenguin at 2:32 PM on October 20, 2010

I had a subaru with air intake issues... (broken intake pipe), easy to fix....
posted by HuronBob at 3:17 PM on October 20, 2010

The Meineke mechanic also found and showed me that the strut on one of my back wheels is in fairly bad shape. Its complement is fine but their policy is to replace struts in pairs only.

I missed this. Please take it with all teh gravitas that you may have gathered from my experience with cars that replacing stuts or shocks (and to a lesser extent springs) in singles is ridiculously dangerous to the point of criminal negligence. Absolutely do not, under any circumstances, consider replacing just one.

It's actually far better to replace all 4, but doing one pair at a time will only give you a crappy ride as a likely consequence, but a mismatched pair of shocks across one axle is almost guaranteed to give you dangerous vehicle handling.

Seriously, without saying "Don't fucking do it", I can't be strong enough about this. Shocks = pairs only.
posted by Brockles at 3:32 PM on October 20, 2010

Duly noted! The mechanic stressed the point more strongly than I suggested, but I tend to take mechanics' advice with a grain of salt when it concerns multiple hundreds of my dollars. ;-)

I'll make sure I replace the struts correctly when I do it, which is to say however long I can get away with not doing it.
posted by The Winsome Parker Lewis at 3:39 PM on October 20, 2010

Just want to mention that "however long I can get away with it" will probably result in you going around a bend on the highway and after hitting a small bump, you will either instanty be in the next lane or on the shoulder. Better to replace struts before they become dangerous.
posted by WhiteWhale at 7:50 PM on October 20, 2010

sanka, those stupid heat shields have gotten loose on every car I have owned. Grrr...
posted by wenestvedt at 7:19 AM on October 21, 2010