AC makes a knocking sound

[MD5150]

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

 

[Kilgore Trout]

Sounds like the condenser is slamming against the firewall when the compressor cycles on and off. Take it back to the shop.

 

[wmburns]

Does it only happen when the AC is on? Could it be something hitting the Fan? Should be easy to rule out by turning off the AC but run the fan only.

If the sound does occur with the compressor cycle, 20-30 seconds seams like a quick cycle (unless it is cold outside). After the dash is pulled, the AC would have to be recharged. You would think the AC should be fully charged. If low, this would indicate a leak.

Will the noise occur while running the car in the driveway? If so, look at the AC compressor and see if it happens when the AC clutch engages.

I think you are right to be concerned. The AC tubes are made of soft metal. It wouldn't take much rubbing to wear through. Besides, you paid someone to do the work. I am sure it wasn't cheap.

 

[Kilgore Trout]

My car has always done this its a known issue with the cobras...

 

[MD5150]

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