I guess it's my turn for a HELP I'M AT THE TRACK AND IT BROKE thread
 

Well, just got to VIR and pulling into the overnight spot my car is making this incredible thumping noise. With the AC running it's much better. With the AC off it's much worse. AC is currently working.

Need to get some rest but wondering what everyone might have to say here.

Feel your pain there. Im headed to the track in the morn. Certainly dont want any unusual noises

Maybe the running AC is dampening the issue? ( which is not a good thing)

Obviously its a rotational issue but could be a no. of things. Hoping its just pulley related & not rod knock

Why did this get moved it's a street car that happens to be at track

Just to close the loop, this ended up being an AC compressor pulley. I think some of the ferros in the pulley were dragging on the clutch magnet. It seemed to reduce itself a bit by the time we got to the track the next day so I went out for a yellow lap and it was fine. ~4 turns into the first hot lap the belt (which was new) shredded. Was able to locate a "close to Evo RS" belt size which there are a number of threads with incorrect sizing info out there...

The Evo serp belt is DEFINITELY 6 rib. I BELIEVE the RS belt length should be 57 and 7/8ths of an inch. For the people out there though, I found a 58" belt from NAPA (all they had locally that was close) and ran that for the entire weekend of track sessions plus the drive home with no issues at all.

No idea how to tag threads but... AC delete, Serpentine Belt Size, Accessory Belt Size

Thanks for the info, glad it wasn't serious

Thank you for posting the conclusion