Thanks for your responses, Mr. Goddard and Mr. "Metal." It was a false alarm. The reason I thought the Firehawk was the issue was the correlation with going into turner mode and the disappearance of the interference on the amp. But now I'm thinking the FH was just serving as an "antenna" for the amp. As in my first troubleshooting tests, when I unplugged the FH from the amp the interference noise disappeared, but this time I picked up the cable to the amp and it came back bigtime; I was now the antenna! So something electrical is going tick-tick-tick somewhere at > 250 beats/minute according to my tap tempo metronome. So the saga continues. I'll post whenever I find the source. Thanks for your help, guys.
Oh, my handle, RickinVirginia should now be RickinFlorida. But when in Va, I lived in McLean and Falls Church. And in Maryland, I was on Ft Meade, Annapolis, Columbia and Crofton.