When I hear some other pilots transmitting, there is a very distinctive whining noise on the radio. I don't believe it is environmental noise, because I have flown the same aircraft types and the flight deck doesn't sound like that. Any ideas what's causing it? You must have heard it yourselves.

Pilots.

I keed, I keed.

Side tone, perhaps?

https://en.m.wikipedia.org/wiki/Sidetone

Poorly shielded antennas and electrical leads.

Often it is old Mad Dogs and their 400hz AC (CSD) Generators "hum"which gets picked up from the interphone system and/or headset wiring...

Which is poorly shielded wiring, poorly routed wiring, antennas, and connectors. That problem can be alleviated with noise filters.

Guys without a contract.........

Thanks for the replies, folks (the serious and the funny ones). Much obliged.

Lol, exactly what I would have said!