Many thanks for the helpful replies folks.
I just removed R919 and it measured as almost exactly 10K. I don't have any 9.1K or 11K resistors so a put back in a 10K paralleled with a 1M making 9.09K. This didn't change anything.
I don't know what damping substance Tektronix used but I would think that high temperature RTV sealant would be suitable. Do you think I should give this a try?
Holding the probe near T906 I get a very clear waveform with a period of about 48uS which I believe gives a frequency of about 20.8kHz. Obviously that's not the loud audible frequency I can hear though as 20K is too high. However, half that (i.e. about 10,000Hz) would make sense. I just tried playing about with a test tone generator and I estimate the noise the scope is making is indeed about 10,000Hz
I have a frequency counter on my multimeter. Please can someone tell me where best to connect the leads of the multimeter to measure the frequency the HVO.