loud pop

[tmacman]

I,have a 78 911sc 3.0 started it up heard a pop then what I thought was an exhaust leak pretty loud, could not find one i'm thinking exhaust gasket or head gasket,car runs but the idle is up & down,it has ss heat exchangers any ideas,tmacman

[Jack Stands]

Do you have a blowoff valve in the air box? Could have been a backfire.

[Flat6pac]

Check airbox for blowing. look at the little screws around the perimeter and see if they have come loose. If you have a pop off valve, make sure you didnt blow it out of it location. Check the rubber seal under the cap/
Bruce