[HAM] 147 "helicopter" noise at low volume

[Hugh]

I'm working on a 147 with unknown prior history that works reasonably 
well but certain frequency pairs, especially low frequencies like say 
G2 and B2 together from an analog synth creates a "helicopter" noise 
about as loud as the tones. I've swapped out the amp with a 125 amp 
and the problem disappears. I've also swapped out the 12AU7 which is 
the only spare tube I have.

Any suggestions on what to check first?  I've already recapped all 
the electrolytics and measured the resistors for obvious 
discrepancies and found none. I'm guessing it's an untamed 
oscillation somewhere, but where?

Thanks,
Hugh