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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?

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  • Is this a mechanical noise, or perhaps a slight blip heard over the radio speakers? Are you sure you have the frequencies correct? 100 clicks / second would just be a flat tone noise. Possible hydraulic lifter noise? Have you done the monster big screwdriver test (MBST) yet? To do the MBST, grab your biggest longest screwdriver from your tool box. Place car in park, open hood, leave engine running. CAREFULLY place the metal end of the screw driver against the top of the rocker arm cover place the handle of the screw driver firmly into your ear. Listen for the noise.
    – zipzit
    Commented Apr 25, 2016 at 10:59
  • Safety, Safety Safety! while doing this test. Don't try this at night, don't try this with out an assistant if you have a stick shift, etc... Move the end of the screwdriver around on top of the engine to see if the noise changes. If you must you can also try a stethoscope, but I think the screw driver works better. Again please be careful of moving fan, fan belts, alternator etc... Note: my Monster Big Screw Driver is about two feet long. Don't try this with a small screwdriver, too easy to hurt yourself.
    – zipzit
    Commented Apr 25, 2016 at 11:04
  • @zipzit Manhy of the OP's here may not know your terminology or howto do the screwdriver listen trick. Sometimes we need to educate them regarding what to do in order to discover root cause.
    – DucatiKiller
    Commented Apr 26, 2016 at 6:54
  • Is the ticking sound in direct correlation with engine RPM?
    – DucatiKiller
    Commented Apr 27, 2016 at 4:39
  • is it a ticking sound similar in anyway to if your hazards or blinker were on? Or more of a card in spoke sound? Sorry can u clarify the type of ticking at all?
    – Honda_Girl
    Commented Dec 11, 2023 at 15:49

1 Answer 1

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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.

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  • Interesting - any idea what is vacuum operated in an HVAC system? I thought it might be the engine mounts as they were shot, but the noise was still there (less noticeable) after replacing them. I've just learnt to ignore the sound now, as it's quite faint...
    – tgun926
    Commented Jan 5, 2017 at 21:03
  • @tgun926 - The blend doors might be vacuum actuated. These are the doors which control the hot/cold or inside/outside air. They also direct the air to different parts of the vehicle, like the floorboard or the windshield. Mind you, this is just a spitball of an answer. Most cars today use motor operated blend doors.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 5, 2017 at 21:28

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