Vibration/hum After Timing Belt

[UWAM]

I just changed my timing belt, idler and tensioner pulleys. It runs fine, and sounds fine at idle, but over about 4, 4.5k RPM there's a significant humming noise and vibration.

What could it be? Is it normal for a belt to make that noise when it's new? It's just a normal nissan belt. The only other thing I can think of is the tension being wrong, but we spent a while doing it, and my mechanic mate reckoned it was alright.

Suggestions?

Cheers

[UWAM]

It only seems to do it under load as well.

[Moodles2]

Was the balancer damaged when it was taken off (notice any cracking?)

[UWAM]

We chipped a bit off one of the ridges while taking it out. It's only the light stuff, it probably weighs 20 grams, which we didn't think would make a difference to the balance of something that weighs many times more. Also thought if it was that it would be noticeable when revving it stationary.

Could that be it?

[Birds]

Ouch, could be anything really...when you change a part like this it often brings out different sounds from the moving parts it interacts with because you're resetting positions that have evolved with each other over time. Could be a loose bolt, bad pulley (water pump replaced too?)...

[Moodles2]

The harmonics of a straight 6 gets worse as the revs climb, I would imagine revving while stationary wouldn't be putting the load onto the crank you would normally get while driving. If I damaged my harmonic balancer in any way, personally I would have binned it and replaced it with a aftermarket balancer. The worse thing that can happen is the oil pump gear breaking in some way.

[UWAM]

Think I found what it is. The had a look at the fan, and there seems to be a fair amount of play in it. You can visibly see it vibrating when it's running, so it's probably that. I'll take it off and run it again to see if it goes away when I get a chance.

Cheers