I wanted to share my recent troubleshooting experience with my home-built Dynaco ST-70 (one of Bob Latino's kits) should anyone else encounter this problem. After a couple of years of perfect operation, I started hearing a persistent high pitched whine coming from the unit itself and some speaker crackling after my unit had warmed up. Infrequent at first, but after a few months, the noise would consistently appear just a few minutes after powering up the st70. Another symptom was the center transformer making a separate sort of resonant tone sound that you could dampen by touching it with your finger. After lots of probing with the multimeter, I finally found the root of the problem.
Short answer: It was a bad 5AR4 rectifier tube. The main noise component appeared to have been coming from the 5AR4 tube filament.
The fix: I replaced the Chinese-made 5AR4 with a Telefunken GZ34/5AR4, readjusted the speaker biases, and my st70 is back in perfect operation. Moral of the story: Stay away from cheap Chinese tubes.