alternator sound

[turbotravis]

i recently installed a serpintine set up on my 67 the donor was a 95 suburban. the new alt has one wire that plugs in and one big wire that went to the battery. i figured the plug in wire was the exiter wire so i hooked it up to switched 12 v. the alt is charging fine but when the key is off and the truck is not running you can hear a faint squeel or eeeeeeeeeeeeee..lol i have been disconecting the battery scince i heard this because i figure it cant be goo. why would the alt make noise??