1999 Regal with 2004 engine

I have finished installing the 2004 engine in the 1999 Regal. Everything bolted right in, the only adapter needed was for the throttle housing. The car runs great. With the aluminum intake manifold of the 2004 engine I will no longer need to be concerned about the problems associated with the plastic intake manifold original to the car. The only minor problem now is a constant knocking sound whenever the ignition key is on. The sound seems to be coming from inside the car, from behind the center of the dashboard. I suspect the noise is associated with the heater/AC components. Any ideas what might be the cause? Thanks in advance

Can you better define the sound? Is it more like relay chatter or a motor type sound. In the center area of the dash are the two temperature actuators if your problem is HVAC related. Manually adjust the driver and passenger temperature controls on the climate control unit and see if the noise changes.

Thanks for replying. I removed the fuse for the HVAC and the knocking sound continues, so now I don't think it has anything to do with the HVAC. The knock is at a constant pace, with about a beat per second. Sort of sounds like a rubber band being pulled and released with it hitting on a plastic. Maybe a solenoid? But I don't think there are any solenoids located in the area behind the radio. Are there?

Was that the fuse to the climate control system or blower motor?

There are vacuum actuators operated by engine accessory vacuum to control the vents and the two temperature actuators which are motor driven which could give a synchronous noise. Also, the RCDLR receiver module is in that general area.

What happens if you remove HVAC accessory vacuum located at the rear of the UIM. The line routes around the firewall and by the battery before going into the firewall. Is the climate control system working 100%?

The knocking sound was coming from the left temperature actuator (there are 2 fuses associated with the HVAC). I disconnected it and now the AC works fine, although only in manual mode-- I can live with that :) Thanks bnaylor