I've been working through a 30L-1 I inherited and have replaced the HV board and installed new Cetron tubes. It worked fine - loaded like it should with 500+ watts out on all bands. I didn't use it for a few months but put it back in line a few days ago. I ran through the testing on all bands and when I got to 15m it would start doing this faint "squeal". This is happening when the bandswitch and tuning control are set to 21Mhz, with the amp at idle & transit not engaged. I switched to 28Mhz and it does the same thing there when I tune the "tuning cap" past the 28Mhz area. If I move the tuning control away from the corresponding band, the squeal stops. So, this is happening with the amp idle, transmit not engaged. I didn't attempt transmit on these two bands. When this faint squeal starts, it throws a strong noise level into the connected transceiver. I did some cleaning on the relay this morning and didn't see anything that looks corrosive or dirty. I did see the grid resistors are due for replacement, however I doubt that's the issue. Anyone have thoughts on this one?