So I have a 01 mustang v6 auto with a knocking/clunking. It only happens when the car is in drive and idle is at 900-2000, when I put it in park, the noise dies down so quiet you can barely hear it or sometimes you can't hear it at all. The oil level is perfect, doesn't matter what octane gas I use and I just changed spark plugs (even though the noise seemed like it happened around the time I changed the spark plugs). Any ideas what this could be?

 

Trying to diagnose a noise thru a forum is almost impossible. To help, try to be as specific as possible. Is it a loud knock, like metal on metal, or tinny like a heat shield vibrating. Is it rpm specific, increases with engine rpm.

Since you said it's only gear that it does this, have a friend or you, poke around under the car with the the other persons foot on the brake while in gear. I don't mean go crawling underneath, but just listen to get a better idea of where it's coming from. But I'd start with the exhaust heat shields. They rust off and rattle , usually only heard at idle.

 

I agree with 88 orange. I would even suggest taking it to a reputable shop before it gets worse.

 

If your car is knocking, maybe you should answer.

Duh!

 

Its a midly loud knock ( not super loud like oh my god its gonna blow, but enough to be heard with windows down), its honestly sounds like solid metal "clacking" on each other, kind of like what a thrown rod sounds like but I'm pretty sure that's not it as I've seen plenty of cars with thrown rods or bearings on there way out. The noise only happens only when gas is given, not heard at idle. Its repetitive, goes in sync with the cylinder fires. I will check heat shields, but are there any ways to check if rod, bearing, or lifters are going bad? Like compression test? I just don't want it to be something with the internals.

 

Your description doesn't sound good, if it's a solid metal on metal knock,that follows engine rpm. You can skip the heat shields, that's not it. I'd start by getting a good oil pressure reading, if your bearings are that gone, you'll have very little to almost no oil pressure at idle on a warm motor.

EDIT: Blown exhaust manifold gaskets may be something to check also.

 

Sounds good, I'm gonna do an oil pressure test and post results.

 

After checking for good oil pressure,you might try pulling the spark plug wires individually,to see if you isolate the sound if it is internal.

 

OK so say if something was wrong internally with cylinder #2 for example, if I pulled cylinder #2 plug, the noise will go away? Wont the car just die?

 

Also, wouldn't a bad bearing make noise all the way through a rev and get louder? Mine goes away after about 1500 rpms.