worf928

This noise does not sound like busted diaphragms to me. It is too mechanical-sounding.

Also, with the engine not running, the vacuum reservoir will 'empty' quickly with a split diaphragm and thus the noise would cease after a minute or two.

A quick test would be to pull the blower relay(*) and see if the noise goes away. If it does then it's not the diaphragms.

(*) MY of 928 not provided. I checked the '87 wiring diagram - quickly - and don't think the blower relay supplies all power to the HVAC components. But, I could be wrong.

2002M3Drew

So if I understand you correctly, the actuators will still make noise if they are the culprit, even if the fan blower is off. Is that right? I think I'll start with this test and see how that goes.

dr bob

Drew-

The connector for the blower resistor pack is on the forward face of the firewall, but under the plastic cowl cover, a bit to the left (driver's side on US cars) of center. Pull that to disable and rule out an intermittent fan motor as a possible cause. Disabling the fan this way leaves all the other HVAC system functions undisturbed.

worf928

Pull the blower relay and see if the noise stops. If it doesn't stop, put the blower relay back in and do what Dr B describes above. If in both of those cases the noise does not stop then I guess it must be the vacuum actuators. I simply find it difficult to believe that the actuators will make the noise that's in your video.

Also, the two tests are very easy to do.

dr bob

Dave-

The diaphragm actuator for the comb flap seems to tear/split at a roll/flex point inside the can. The size of the lines and the little solenoids limits how fast air is drawn out of the actuator can. When the solenoid is actuated, the diaphragm actually holds vacuum until it moves enough for it to fold over at the split. That opens the split and "dumps" the vacuum in the little actuator canister all at once. Then it draws back again relatively slowly. The clicking happens each time the actuator dumps and the comb flap closes again. Mine made exactly that noise, plus the AC wasn't very cold, when my comb flap actuator failed. Black DD car with no tint in SoCal forced an expedited repair.

2002M3Drew

So some time has passed and I still haven't figured this out. I have found, however, that as long as I set the heat setting to somewhere above the ambient temperature (i.e. to keep the heat on), the car doesn't make the noise. If I set the heat slider all the way to the right even on a warm day, the noise won't happen. If I sent the temperature at or below ambient, it will eventually make the noise (sounds like a vacuum stays open when the car is delivering heat, and that it makes noise when it attempts to close).

dr bob

Drew --

The temp control is masterminded by a little servo that drives a diverter flap between hot and cold, and also includes some smarts in the form of switches. The servo follows a balance among the tem sensor in the dash, the temp sensor for ambient air (in the alternator cooling duct under the left headlight...) and the temp slider setting in the HVAC controller in the dash. The little servo is above your throttle foot if that helps. The servo actuates the various solenoids and flaps to control airflow, and also manages the heater control valve when heat is needed.

The workshop manual has a pretty good "truth table" that identifies which flaps move when the various controls and temperature balances are present. Start with that, and manipulate the controls and settings to identify which are active, and from that at least narrow down the selection set a bit. Then pinpoint by location. The solenoids themselves do fail per testimony here, so pull that left-side console cover as you go through the test sequences and see if one of those is actually making the noises.

uraniummetallurgist

I had exactly the same loud clicking develop a couple of days ago. The only way to make it go away is to turn on the fan. The clicking immediately stops. Positioning the the airflow selector or temperature selector doesn't do anything. It's a PITA but OK however, I'd like to get to the bottom of this and sort it out.