AC makes a knocking sound

2001 GT

I just got my heater core fixed at the Ford Dealership and now whenever my AC is running for a few minutes it starts making a knocking sound and continues on about every 20-30 seconds. Sometimes it gets a bit loud too. I would describe it as a single muffled knock....like a small wrench being hit against a fender or firewall. It is clearly coming from deep inside the dash, or down there somewhere. When it knocks I can feel it in my floor pedals. But the sound itself sounds more like it's coming from the passenger's side deep in the dash, or down in the upper floor board area. It only happens when the AC is running. And it definitely never use to do this before the heater core was replaced. However, everthing is actually working in regards to AC/Heat. But that sound really concerns me.

Any ideas?

Thanks,

Mike

Figured it out..

Yes, wmburns, only when the AC is on.

But today I found the problem! It's actually coming from under the hood near the firewall. One of the AC tubes is hitting my caster/camber plate. I guess after the heater core job things are a bit tighter in that area and now when the compressor cycles goes off it jolts the line making it hit the cc plate. I'm so glad this is an easy fix and nothing serious.

Thanks to all for your input.

Mike