Weird 'Clicking' or 'Ticking' sound from behind the dash.

[Bobthecarguy]

I have a strange ticking sound that seems to be coming from behind the glovebox inside the dash. The car is a '98 Mustang GT convertible and it doesn't do it in park but only under load. Very light acceleration doesn't cause it and heavy acceleration doesn't do it either. In summer weather it sometimes happens after about a half hour drive on the highway but not every time and in colder weather it seems to start as soon as the car is warmed up. Someone in another forum simply said 'header leak' when I asked there but it isn't that. My brother is a licensed mechanic and suggested something in the heater system, maybe an actuator?

Any help would be greatly appreciated. Thanks in advance.

Bob.