My Belles 150 is buzzing when I turn it on..

Is this normal? I just bought this amp used from someone, and although I don't have speakers yet, I've tried turning it on and what I hear is a vibrating/buzzing noise right when I turn it on. The noise then gets quieter, but remains constant.

I don't think this is normal, but I want to check with you guys before I take this up w/the seller.