I have a humming after I turn the car off too...never really looked to see what it is as my car runs fine, so I figured it was normal...and I still figure that.
Squeal? Yes, as mentioned, if you didn't replace all three pulleys (2 tensioner and 1 idler) then that might be your squeal source.
Pulleys can be installed at a slight angle...if idler's pin doesn't fit in the notch, it'll be off. Any of the other pulleys might be bad too...doublecheck them when the belt is off (PS, ALT, and WP). Use a hose as stethoscope to isolate squeal if you can...as you want to make sure it's not stuff you really need...like alt and wp. Usually the pulleys on these are fine, but in particular with the wp, if it goes, you'll be stranded or worse if you don't know what to do when the car starts to overheat.
Look well. I don't think your humming is an issue...but the squealing sounds like pulley to me. Replace your belts if you don't know they've been done in the last couple of years. Also, when you put the belt back, did you put it back in the same direction? If not, that could cause squealing right there I think.
I don't want to beg, but please, please come to a show sometime!
In NYC Fri, Sept. 30, 8 p.m. at Broadway Comedy Club! Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Mangofy your E46!