Well I had the same problem on my 66 and it turned out to be the connection to the exhaust manifold. I have Clifford custom headers on it now so it's not there anymore. The old stock manifold was just worn and tired and would never quite cinch up to the exhaust pipe (even with a new thick gasket). Check all the bolts real close as they can look tight even if they're not. Work the throttle linkage a couple of times while you're listening around the manifold and you should hear the rumbling/ popping noise. I would think if it was a head gasket you would notice coolant in your exhaust.
Hope you figure it out.
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!