What is this noise???

In the video below you will hear the rattle when the rpm is between 2k-3.5k rpm. Its very prominent, as the rpm falls after I let my foot off the gas.
Sounds like its coming from underneath the car.

video link -> http://flic.kr/p/a25FaQ

I dont think it has anything to do w clutch/flywheel.
Blown exhaust gasket? Heat shield rattle? Something else?

I have had resonated test pipes for couple of months now and this noise just started this morning.

 

Sounds like a loose heatshield or parts of the cat have fallen out and are rattling in the exhaust.

 

^ where is this heat shield?
I have test pipes.

 

They run the length of the car in between the car and exhaust.

 

definitely your exhaust something lol

 

Had the same problem when I installed my test pipes. I'm almost sure it was from the flex pipe.

 

^ i dont think so, cuse i have had the test pipes on for 2 months and this noise just started this morning.

 

Just jack up your car and get underneath or have Jake do it for you. Maybe the bolts have slightly loosened and causing a rattle or the heat shield. Noises can appear overtime even when during install it was perfect. Your motor is a revup have you still been burning oil since the new one.

 

Im pretty sure its not that... I know that sound pretty well... lol
Thats a continuous knocking sound coming from engine area, whereas this is a rattle sound between 2k-3k rpm coming from underneath the car.

 

Not the motor but if your motor is burning oil it could work its way into the CATS.

 

Definitely with your exhaust. I would bet money it is something right around your test pipes. Check the heat shields as Dave suggested. Maybe a bad gasket. Possibly the flex joints in the Y.

 

I had this problem. Mine was the two bolts where the cat brace connected to. If you have test pipes and didnt but the cat brace back on, those are just rattling around. Just get two nuts and make sure those bolts are tight.

 

Ok, so I got underneath the car... and noticed 2 things...
1. My driver side o2 sensor had come lose... i can hand tighten it and hand loosen it as for right now.
2. There is a heat shield above my test pipes on both sides, and the driver side has a paper thin gap bet the 2.

I held the test pipes and shook them around, everything seems tight.... after shaking stuff around for about 30mins, lol, went for a drive and the rattle noise has reduced.
As we all know its like 200deg outside and that expands metal, so I believe its one of the above thats causing the noise.

But, I still need somebody to look at this, cuse I could be wrong!!!

 

Sounds like you are on the right path.. I would get under the car and make sure everything is good and tight.

 

It sounds exactly like mine did a week after I put my pipes on. My O2 sensor was also loose. Just grab a pair of channel locks or even a vice grip (if you dont have the right wrench that is), and tighten it up. I'd check both of them.
A month after that, I got a similar noise again. I lifted it up and found a bolt loose on the upper TP flange. I guess I missed that one when I put thread lock on the bolts during the install. You said that you thought it wasn't a bolt because you've "heard that sound before", but when the bolt is on the exhaust it will only vibrate at certain RPM's because of the resonation/vibration at certain rpm's...just like the O2 sensor did. Good luck