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Check all your exhaust bolts are tight, in fact any bolt attached to something that could rattle at a given RPM.
I had a similar issue on an previous car that I'd just had rebuilt. Heard the noise, felt like I'd been sucker punched in the stomach, then got under the car and found an exhaust bolt loose which was rattling right around 2500rpm. I thought I'd spun a shell...
It sounds external to me but i really had to crank up the volume.
I agree with Sam jack it up and give every bolt within reach a twiddle to see if its spinnable. Something sounds loose.
There's a bracket attached to the tranny bell housing that bolts to an exhaust bracket. Easy to get to from either top (under the hood) or underneath. Check that first.
I'm running a MK turbo kit, so no downpipe to bell housing bracket, and no exhaust bolts (just v bands). I'm gonna take a really close look at pulleys. I have no AC or PS, so I'm going to look closely at crank, WP, & alt pulleys.
This noise puzzles me. It's not quite a heat shield rattle type resonant buzz, and it doesn't quite sound engine internal.
I once replaced a short block because what I thought was rod knock in an unknown condition bottom end was actually just the exhaust banging on a jack stand... check for loose bolts and other sources of vibration. If you find nothing, pop the valve cover and look for damage, then if nothing, drain the oil and look for glitter.
Yeah, that's the plan. Now that I can replicate the noise in neutral, I can just rev it in the engine bay & listen for the horror.
It really sounds like maybe a dust cover bolt hitting the crank pulley or something, but I just haven't found it yet. I'm really hoping it's not engine internal. Or even inside the bell housing for that matter.
I only had a couple minutes outside to poke around the engine bay, but a conspiracy is unfolding. The lower timing dust cover is broken at one mounting location, and the back side of the timing wheel is polished to a mirror finish. There's some molten plastic shreds around that almost look like spider webs. I think it might be the teeth on the timing wheel making that noise.
Well, I wish I could say my problem is gone, but it isn't. After replacing the timing cover, I fired up the car and revved it a bit. No noise! But today I took it for a drive, and it starts this racket. It seems to only do it when it's up to temp. It isn't the VVT. I unplugged the oil control valve, and it hasn't gone away.
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