Quad 909 oscillating. Why?

Something strange is happening with my Quad 909. It had been working fine but the other day I turned it on and even with no musical signal going through it and even with the preamp turned off I heard a loud oscillating sound coming out of my speakers. I immediately turned it off, grateful that it wasn't hooked up to my Quad 2805s when that happened.

Any idea what might cause this? I won't be trying to fix it myself but am thinking that it's a good idea to have a sense of what might be wrong before I take it in for a repair.