i've had my vox ac15 tube amp for about a year now and it's never had any problems until recently. i plugged it in today and played for about 5 min and then it started making these high-pitched squealing noises. i tried unplugging all all my pedals and just plugging the guitar directly into the amp but it's still making the squealing noises. does anyone know what the problem could be?
Microphonic preamp tube maybe?

Buy one preamp tube and swap it out with your current preamp tubes until you find the culprit.
^ they are probably right. The combination of the tubes being in use for about a year and your amp being a Vox probably killed the tubes. Absolutely amazing amps, but it and the AC30 are notorious for short tube life