[Nuvistor]

The three valves all have their heater connection on pins 4 and 5, the ECL86 have two valves in each envelope, triode and pentode, their heaters are connected to pins 4 and 5. Your meter on AC volts 10v or similar setting depending on what ranges your meter as and test across pins 4 and 5 on all three valves.
The rectifier V3 is the same but it has its own heater winding on the trannsforner.

The “secondaries “ this is the supply to the anodes of the rectifier, meter set to AC volts range 1000v. One meter lead to chassis the other taking separate readings on pins 1 and 7, both should be in the range of 250VAC.

I will be surprised if there is much wrong with them.

Need more information on the buzzing.