Ticking noise which gets faster at idle and slows after blipping

Question

Car: Audi S5 2008 V8 4.2L

Transmission: Manual

Symptoms: There is a quiet (but noticable) ticking noise which can be heard from inside the cabin when the car starts. It starts of slow (maybe 10 clicks/sec) and starts getting faster and faster (maybe 100 clicks / sec) and remains. It's sort of like a positive feedback loop. When I blip the throttle and let it idle again (to around 2k - 3k rpm), the clicking speed resets itself, and starts slow and speeds up.

Notes:

1. It's not the direct injection solenoid valves

2. The noise cannot be heard above idle

3. The noise can only be heard from inside the cabin (too loud outside)

4. The noise remains after the car has warmed up

Any ideas?

Answer 1

From what you posted in your question, this would seem to me to be something in the HVAC system which is vacuum operated (taking a stab at this). Since it is not dependent upon engine speed for the noise (you stated the noise increased speed while the engine itself remained at idle). You also stated you can only hear it from inside the cabin.

My thinking is, if this is a vacuum operated something in the HVAC system, as vacuum builds inside the system, the clicking increases in speed. This could easily be so if the HVAC system has a reservoir associated with it. Once you blip the idle up to a running speed, the vacuum reservoir is depleted and the cycle starts all over again.