There is a squeaking noise that I first thought was valvetrain noise but today I was under the car while it was running and it seems to be coming from the crank pulley. What could this be?

 

Typically it's a worn accessory belt. To confirm it's the belt, but a can of belt dressing and spray it on the belt. If the noise goes away, the belt is the problem. Belt dressing doesn't solve the issue as the squeek will come back. Replace the belt.

 

The belt has maybe 4k miles on it. I had it replaced when I had the motor completely rebuilt.

 

Do NOT use belt dressing on a serpentine belt...It will make it squeel.

 

I really don't think its the belt. What else could it be?