Speaker noise when turning on amp?

Hi. I just purchased a used McCormack DNA-125. When I turn on the amp, it makes the speakers hum quite loudly. I can see the woofers on the speakers oscillating after I flip the rocker switch, and a pretty loud hum ensues for about 1/2 of a second. I did not have this occurance with my other amp.

1) is this normal?
2) will this damage the speakers?
Sounds like a ground loop hum. Try this site, I've found it helpful. Click on the Humbuster link on the right. You'll have to become a member, no big deal.

PS Audio Toolbox

It will walk you through your problem step by step.

Good luck,