Belt slap

We have just fitted a new crank pully, new tensioner and pully, new idler pully, new belts and 2nd hand alternator.

All runs well and alternator is charging good. But when start engine there is a noise at idle. Upon closer inspection the belt at the bottom from crank pully to alternator is flapping up and down hitting the idler. After a few mins it stops. If slowly rev the car whilst stationary it flaps again but doesn't do this whilst driving. When engine is off the belt all feels nice and not loose etc. Could this be the alternator pully? Or something else. Driving me mad.

Any help be very much appreciated

Probably belt and pulleys grabbing a bit till they bed in, sprinkle some talcum powder in the belt

Is the tensioner adequately tight? Did it take 2 hands to pull up the spanner on the nut?

Never seen one slap..shows it is loose and might jump off.

Yes took 2 hands pull tensioner. It stopped when wd40 was sprayed on the belt. But then comes back. Only does it on idle. Or if I rev a little. It drives all OK and no noise when driving

freelance said:

Probably belt and pulleys grabbing a bit till they bed in, sprinkle some talcum powder in the belt

Click to expand...

I try the talc powder? What will that do?

A dry lubricant