Exhaust leak?

[FFRBob]

I've got a fluttering noise that's starting to get more noticeable. The 390 FE is newly rebuilt with a hydraulic flat tappet cam. I broke in the cam with crane cams break in lube and 30wt Rotella by running the car for 10 mins at 2000 rpm. I had to stop to let it cool down, then ran it for 5 more minutes, let it cool down and a final 5 minutes all at 2k rpm. The timing is set at 12 degrees initial with 24 total advance. I did an oil change immediately after break in and didn't notice anything unusual in the oil, but I did not cut open the filter.

I now have about 30 miles on the motor and I'm starting to get a fluttering noise from the engine compartment; it doesn't sound like it's coming from the pipes. It's more obvious when the car is warm and it increases with the rpms. Please say this sounds like an exhaust leak and not a flattened lobe It's definately not a ticking noise and the car doesn't backfire. I do notice a bit of an initial flat spot when I stab the gas to half throttle, but then it takes off like a rocket. I jetted the carb down one size, but I think I need to jet down one more since I'm at 3500 feet elevation and the plugs were pretty black.

Anyway, help me FE gurus. Thanks in advance.

FFR Bob