Okay, I have an original '68 Ampeg Gemini 1x12 all-tube combo.

Lately I have noticed this when turning it off. Theres this odd growling noise.

I'll throw the stand-by, then turn the power switch off. When I do, it sounds like a distorted gggrrrrRRRRRRRrrrrrgggg. Like a distorted, flanged, growl. (and this is without any pedals in my chain, just guitar-->>amp.) I would post a sound clip but dont have anything to record with.

Anyway, based on the info, what would most likely be the problem? Or does this happen just with age?
Gibson Les Paul Custom
Fender American Tele

Orange Rockerverb 50
Orange PPC412
I dont think so. I dont smell any burnt dielectric or anything. Although that doesnt rule caps out by any stretch.

Could it be rectifier or weak tubes?
Don't try and touch anything inside the amp. You could dieee

Take it into a technician. Messing with tube amps is never a good idea.