Faulty Sub, Cable or Receiver?

[Khrest]

Looking for ideas how I can diagnose where this fault seems to lie. My Receiver is a year old Denon, the sub is a 14 year old Yamaha with the original cable running under the flooring in our lounge.

For the last week or two we've had an issue where the sub just starts to emit a rising tone. This increases until it's enough to shake the house (it actually did it once at 4am, now we remember to switch it off before going to bed!) and then if you allow it to run it's course there is a loud pop-off sound and it goes quiet again.

It will emit the tone whether we are watching something or when the Receiver is powered off. Pulling the cable connecting to the receiver stops the tone, but without a knowledge of how these things work I can't work out if this means the receiver is emitting the tone or the sub is generating some kind of feedback loop inside.

 

[Khrest]

Wow, thanks for taking the time to write that matey. I'll have a good read through and try some of the things you suggest. I don't mind having to buy a new set of speakers (after 13 years!), but I would hate to buy them and then find I still have the problems because it was AMP or cable.
To answer your question, the cable is under wood floor, so it's sandwiched between the concrete floor of the room and the underlay for the wood flooring.

 

[Khrest]

So to update this story... Moved the sub across the room so it could be connected with a different cable. Worked fine until Friday night so I decided to move it back and reattach the regular cable. Over the weekend it's worked fine. So until I can reproduce the fault with the old cable connected the jury is still out; seems as though removing and reconnecting OR moving the hub around has sorted it for now. Thanks for all the help, at least I know the next steps if the problem returns and the new cable doesn't fix it again.