Okay, engine mounts or exhaust leak? (And if so, where?)

[leapdragon]

Hi all,


So over the last 3-4 days my '07 XC70 has developed a loud, deep, throaty rumble/resonance on acceleration and a new, minor, medium-pitched drone sound on the highway while in cruise.

At idle, car sounds somewhat normal, though it's just possible it's slightly louder than it used to be. Revving at idle, same—nothing significant but I feel like there might be more of a bass note to the engine drone than there used to be.

Put into drive and give it gentle gas, same.

Put it into drive and give it more than gentle gas, it sounds honestly like a car with a leaky resonator or muffler or something. Very "engine forward," big bass notes that you feel in your bones, getting louder the more gas you give it.

Put it into neutral and coast, and it all seems to go away and we roll more or less silently.

Obvious candidates are:

- Engine mounts
- Exhaust leak
- Drive shaft
- Wheel bearings
- CV joints

Problems with each candidate:

- Mounts: Engine mounts are newish (20k) and I can't rock the engine at all with my hands
- Exhaust: I can't see any obvious signs of a leak and while running at idle can't hear any hissing anywhere
- Drive shaft: Also newish (10k) and when checked by hand feels nice and tight and smooth
- Wheel bearings: Also newish (20k) and you'd expect them not to go silent if rolling in neutral
- CV joints: I guess I just wouldn't expect them to make this kind of deep resonating growl

I have opened the windows up next to a couple long sound walls and accelerated hard along them. I don't hear any major noise coming from outside. In fact, in general, it feels like it's louder inside than outside. I'd say the source location "feels" like it's just behind the heater core, if that makes any sense. About the area where the turbo sits, though of course trying to figure out "where" a sound originates is a fool's game.

If I had to bet, I'd bet it's either the engine mounts or the exhaust system somewhere. But I hate to just start replacing things because exhaust parts are expensive and the job will be a mild PITA and because engine mounts are newish. I know P2s can be deceptive, with an "exhaust leak" that turns out to be bad engine mounts—but do engine mounts really go bad after 20k? They seem visually OK and like I mentioned, I can't rock the engine...

Anyway, just wondering if anybody have any clever diagnostic tips/tricks to try to help me figure out what the source of this noise is before I start unbolting and/or replacing things.

[chrism]

Put the car in drive, hold the brake, and apply the throttle. Note the noise.
Then put the car in reverse, hold the brake, and apply the throttle. Note the noise.

If the noise sounds the same in both drive and reverse then I would suspect the exhaust.\ system.
If the the noise is different between drive and reverse then I would suspect motor mounts.

[leapdragon]

That's a great idea, thank you! Will try this afternoon.

[jonesg]

block the tailpipe with tape, poke an air nozzle through the tape and inject low air pressure.
Spray soapy water on joints. I don't think you'll find a leak though.

I would also suspect angle gear.

[Blacklab467]

I was just going to say don't rule out the angle gear, also could be CV joints if it does it under acceleration and its gotten worse in a short period of time. How many miles on the car?

[leapdragon]

Okay, plot twist.

With the forward/reverse but the brake on, I wasn't able to get the noise that I get on the road under acceleration. I had the car on ramps while doing this and the skid plate off and the door open and I don't think I hear any loud exhaust noise.

However—apparently I have not been paying attention to reverse recently. When I put the car into reverse, there is a significant dull, low, loud vibration just sitting there at idle. Here's how I'd score it:

- Idle P — loud-low vibration level 0
- Idle D — loud-low vibration level 1
- D + gas + brake — loud-low vibration level 2-3
- Idle R — loud-low vibration level 4
- R + gas + brake — loud-low vibration level 4-5

Am I wrong in thinking this sounds like engine mounts?

[jonesg]

yeh kinda sounds like mounts.

[leapdragon]

So, follow up, I had FCP Euro rush me a new set of Hutchinson engine mounts. And it appears to have solved the problem. I've only put 10 miles on it so far, but it seems to be very quiet again, no hint of that earthquake-like rumble.

Strange thing, I had put an entirely new set of Hutchinson engine mounts on last summer. And on removal, they all seemed visually okay. The only thing that comes to mind is that I did have a run-in with a pretty serious pothole at the start of this summer and had felt very lucky to (apparently) escape major damage.

Also, for posterity—to get that lower rear engine mount—there is a trick. I started off this time doing the same thing I started off doing last time, which is fighting to get at that bottom bolt. It took me about an hour derping around with trying to pry a socket between the sway bar and the frame and then get a universal joint (i.e. pivot/very wobbly) socket to stay on that bolt before I remembered:

Use your 1/4" set and a 1/4" extension rather than the 3/8" stuff. The 1/4 extension sockets are small enough, and the bars are thin enough that the angles are different... and it slides right in between the sway bar and the frame and goes straight on, without a wobble even needed. The torque spec isn't high enough on that lower bolt to give a 1/4" set any trouble.

If I'd remembered that I ultimately figured that out last time around, I'd have saved myself half the morning repeating the mistakes of last time this go around.

[jonesg]

you solved it by deduction.