i had a similar problem on my 200s , topend was rebuild and every now and then i would get a snapping weird sound when hot. I found out it was the head gasket leaked some because the bolts that held it all together loosened on me. I retorqued them to the proper specs and all was good. I found this out by revving it and i could hear the noise , i had it idleing and i was fliiping the throttle listening to where the noise was comming from , and while i had the noise i saw it was bubbling some from where the gasket was. I retightened and problem was gone ! Good luck
I guess it's kinda like a strong sucking in sound , but also kinda sounds like a snap too.
1985 ATC 250ES Big Red
1985 ATC 350x DG Exhaust , Wiseco 10.25:1 .20 over - Sold
1985 ATC 200s - SOLD