I think I blew the head gasket on my rear right tire...

[heinzboehmer]

Okay, grabbed your attention with that ridiculous title and now need some help with something that I've never come across before.

Today, I noticed that if I put the engine under high load (e.g. 4th gear, 3.5kish rpm and >75% throttle) between 70 and 90 mph, I get a buzzing sound from the rear right wheel area. Sounds like a blown head gasket, but it's obviously not that.

It is definitely speed related, cause I can't get it to happen at slower or faster speeds. BUT, it also only happens at high engine load.

I don't get it. There's nothing engine related in that area (except for a bit of the muffler I guess), so I don't know what it could be. The only thing I can think of is that somehow the battery cable is arcing onto the chassis. However, I took my whole trunk apart and can't see any signs of arcing. Also, if it were that, why would it only happen under those two specific conditions?

Engine is definitely not pinging either.

Managed to take this video of it (had to throw my phone onto the rear right seat to capture the noise):

Any ideas what it might be? I really don't even know where to start.

[heinzboehmer]

Oh also, I thought it might be a speaker glitching out, but the noise happens with the radio on and off, as well as with the volume turned all the way down.

It isn't the wind on the weatherstripping either, as it happens with the windows open or closed.

[Casa de Mesa]

Is it coming through a speaker?

Almost sounds like it's a speaker...

[Casa de Mesa]

Oh also, I thought it might be a speaker glitching out, but the noise happens with the radio on and off, as well as with the volume turned all the way down.

It isn't the wind on the weatherstripping either, as it happens with the windows open or closed.

Legit didn't see this before I posted my comment above.

EDIT: in some cars (not sure if that's the case with these), the radio isn't ever truly off. Also, you can get feedback from stuff in the engine bay - like an alternator. That was my first thought when I heard the sound clip.

[heinzboehmer]

Yeah you're right. Trunk is taken apart, so I'll try unplugging the amp. Presumably with the amp unplugged, the speakers will be fully disconnected as well.

[sapote]

Forget about battery arcing as the voltage only depends on rpm and not engine load.

I think some mechanism loose and vibrated. Throw a 30lbs sand bag on the right rear seat. Also check the exhaust heatshield under the car.

[heinzboehmer]

Forget about battery arcing as the voltage only depends on rpm and not engine load.

I think some mechanism loose and vibrated. Throw a 30lbs sand bag on the right rear seat. Also check the exhaust heatshield under the car.

I'll try both of your suggestions. But why would the noise be dependent on load? Maybe at that rpm the exhaust resonates and vibrates something?

[heinzboehmer]

Okay it's not the speakers. Disconnected the amp and the noise was still there. Guess the next step is to check if there's anything loose under or inside the car.

I'll get to it next week, once I'm back home.

[Jimbo's M]

Years ago I had a noise in the rear (don't remember what) and couldn't figure it out alone so I threw a buddy in the trunk with the seat backs down and drove around while he pinpointed it for me. Whether that'll work for you...

[heinzboehmer]

Years ago I had a noise in the rear (don't remember what) and couldn't figure it out alone so I threw a buddy in the trunk with the seat backs down and drove around while he pinpointed it for me. Whether that'll work for you...

Yeah that's not a bad idea. If nothing is obviously loose, I'll probably do that too.

[sapote]

I'll try both of your suggestions. But why would the noise be dependent on load? Maybe at that rpm the exhaust resonates and vibrates something?

A reciprocate combustion engine will generate more impulse noise magnitude when the combustions work at high pressure (high load). Impulse noise composes of wide frequency spectrum noise and they cause vibration to all other parts on the car, and if the "3-leg" back seat has the same natural frequency response with one of the noise then this become resonance and louder. Same as a loose heat shield or it nearly touch other object, then the resonance frequency could it to vibrate at higher amplitude and touching that object. The suspension also shifted or compressed more under load or accel.

[F1Dryvr]

Just a thought, i had a speed and rpm related groan a while back and got it pinned down after extensive search to the pink driveshaft to diff gasket breaking and all the grease spinning out. Had to disassemble the cv joint and repack and that solved the issue.

[Casa de Mesa]

got it pinned down after extensive search to the pink driveshaft to diff gasket breaking

Those stupid gaskets. First time I saw those things, gave me a good laugh. Pitiful!

[Icecream]

Okay, grabbed your attention with that ridiculous title and now need some help with something that I've never come across before.

The only thing I can think of is that somehow the battery cable is arcing onto the chassis. However, I took my whole trunk apart and can't see any signs of arcing.

.

Did you take out the rear seat cushion? To my surprise, my cable was well charred at that location right were it runs up and into the trunk.

[heinzboehmer]

Did you take out the rear seat cushion? To my surprise, my cable was well charred at that location right were it runs up and into the trunk.

Not yet. But it's definitely one of the first things I'll be checking once I'm back with the car next week.

The charring thing is a bit concerning to hear. It'd be nice to not have the car go up in flames while I'm driving it. Any pics of yours?