Terrible rattling sound
 

Can anyone help me w my z, I get a very very loud rattle right around 3.5 k rpm’s every time. Sometimes at idle too. All heat shields are off the car except the header ones, car has aam test pipes and fast intentions exhaust, I’ve looked at all contact points and can not figure it out. Rattle started the day the resonated test pipes went on the car sounds like it’s coming from front passenger side but I’m not sure, anyone please help!!

Get the car up high enough that you have some elbow room and start "beating" on stuff with a rubber hammer until something rattles. I'd check the exhaust pipes first but YMMV.

I have gone under there so many times this month chasing it lol, I can try and get it higher up to hit it harder, but so far I can only replicate the rattle under load or it will randomly do it at idle

Could be a loose bolt on one of the flanges.

If it happened right after the pipe install then my thoughts would be any and all bolts loosened during procedure. All gaskets replaced I trust as they get squashed and should not really be re-used other than in an emergency. Again like others said it is searching and banging of parts to see if noise starts. Maybe pay a muffler shop to have a quick look.

due to how resonant frequencies work it could be, literally anywhere in the car.
The rear hatch latch mechanism is a cause of common rattling.
You have to open it and fill it with grease to stop it.