My ac unit seems to suddenly be making a knocking noise when the unit is on. It does not make noise when system is off. I thought it might be the fan, but it continues to make noise
While the cooling system runs without the fan.
I can hear the knocking on the Payne AC metal box in my attic which contains the unit. I would appreciate any info you may have. Should I leave the AC off until we can get a service man out. Thanks.

There is usually only one moving part in a indoor airhandler that will cause a noise like you describe. That noise is probably from the fan. It could be a bad motor mount assembly,loose cage fan assembly,or the motor itself. It is always safer to turn the unit off to prevent any additional damage. Without being there it is hard to tell just how bad the noise is so I mentioned to turn it off just to be safe.