360 Cracked manifolds?

Precat failure?

 

Exhaust side for sure: Metal expands with heat and "seals" the fracture.

 

You gotta poke your head around in the engine bay and listen where it's coming from. My tubi exhaust was cracked (not leaking yet) and I just welded it up

A stethoscope or a long screwdriver can help sometimes

 

My exhaust valves are rattling at times when cold.... start her up and put your hand on the valves in the back before they get too hot.

 

Be sure to check the Secondary Air Injection pump. Mine rattles a bit upon start up ( I have 30K miles on my 360) but after a minute or so it cuts off (as it should) and then it's all Ferrari music from there on. It's an emissions thing that is only in play upon a cold start up. I don't give it a second thought. I suppose someday I will address it. We don't have vehicle inspections where I live but it's possible that could be an issue if you did. However I highly doubt it since you would be pulling into the inspection station with a fully "warmed up" engine.

 

Jess, secondary air injection pump is in rear most left (driver) corner of engine bay. If rattle is near glass, that is probably not it.

Get one of these to help you pinpoint the source of the rattle:

https://www.harborfreight.com/mechanics-stethoscope-63691.html

 

Table all possibilities and start the troubleshooting by eliminating one at a time. The easier ones, with the engine on, when I hear the buzzing, put your hand under the intake plenums, it is known that the butterflies linkage rattles from underneath , u could touch it and see if any difference. Also, when headers fail, when u turn off the engine after being fully hot, u will hear metal cracking (louder than normal) also watch for a white like dust everywhere in the engine bay…this is the fiberglass based (or similar) heat insulation being blow out of the heat shields. Last resort, smoke test them to be sure.