knocknocknocknock what's that noise?
 

'88 924S.

I drove the car nine miles this morning to pick up a friend. Nothing unusual happened during the drive.
When I got to his place, I shut the car off, went inside to alert him, and went back out to the car.
I put the key in, and turned to the Aux position for the radio, but did not start the car.

I don't recall whethr it started immediately, or after a few seconds, but after turning the key (and not starting the car) something began knocking at about 300 beats per minute.
It seemed to come from the right front of the car. In hindsight, I should have opened the hood and tried to locate it, but I didn't. I turned the key off, and it stopped. Back on, and it started again. Off, then on... it didn't happen every time.
My friend came out, and we started to drive. No knock. I pulled into a parking lot a couple blocks away to see if it would come back. It did, but only for a second as I was turning into the lot. We waited for a few minutes. Nothing.

It didn't happen on the rest of the way home. After parking, I tried to replicate it. Didn't happen.

After we got home, I did discover the the ground on the battery post was incredibly loose, so I tightened it,

What could this noise have been, with a car that is not started?

ABS pump maybe?

In an '88 924S? Probably not.... no ABS, so far as I know. And I've never felt ABS "kick in" under any braking circumstances.
Pretty sure there's no ABS in that car.

Radiator fan turning and hitting an obstruction.

I thought that could be it as well, but that doesn't appear to be the case...I'm reasonably confident that the fans were off at the time. And the Beats \Per Minute of the knock (about 300bpm) was much slower than the fans spin...even if the fan only had one blade, I'm pretty sure it spins at more than 300RPM.

I just checked around the fans, too... nothing close enough to get whacked.

Now that I'm done with my workday, I'm going to try to repeat it.

small chinese child in the engine bay?

Dunno. I'm starting to think maybe it was a bird flapping around in there or something. I haven't been able to replicate it since then,

Fan
 

I know cooling fans were mentioned and you had good points they do have high and low speed resistors and when the bearings in the fan are failing they can turn slower. I would turn a/c on and turn key on without starting car. That will energize the one fan. The other you can check when it gets warm. The other idea would be the blower motor for the interior

Did that. Nope. Unless the blower motor sits in the front right of the car...nope. definitely was not coming from dash/windshield/cowl area, and the blower wasn't operating at the time, either.

If it is with cruise control, on the inside of the engine compartment you will see outside of the battery-well, mounted behind the right strut the unit itself can be seen very easily. I am willing to say that this is your problem noise. I discounted mine at the plug. Who uses cruise control on a vintage 80's car? I hope this helps.

With the car not running or moving at all? I can't see any reason the CC would make a knocking noise like that...

Years ago, my 944 occasionally did the same thing and I isolated it to the cruise control. I can't even begin to explain how that could be but that's what it was.

I don't think I've had a problem since figuring it out, tho. Maybe I excised that demon?

BB.

antenna motor

My antenna has never moved in the slightest. I don't think it's motorized.