You mite have an exhaust leak when the engines cold and goes away after it warms up due to expansion, but an exhaust leak that goes away by reving the motor once its warmed up is Impossible

Although an exhaust leak is possible on any engine, we have to look at the probabilities here. This is not an MB. The SBC has a VERY weak cam and valve train. The cam is too small, the lifters are too small and the rocker arms are a chincy, stamped piece that a cost conscious engineering group came up with over 50 years ago. Not to mention the original design was conceived to handle no more than a few hundred horsepower.

The SBC modified in '87 for roller lifters got no change in the rocker arm area. They are prone to having noise if the oil is not kept clean. The exhaust manifolds on the other hand are not known for leaking.

At this point we have to operate on probabilities like a physician. The probabilities are HEAVILY in favor of the noise being valve train related.

We also have to give the poster some credit for knowing the difference between valve train noise and an exhaust leak.

Have a great day,