Unusual noise at cold start 1994 940 wagon NA Topic is solved

[Dldowns]

VID_20250811_101634811~2.mp4

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The attached video captures an unusual sound at cold start that seems to activate after engines starts and then literally stops after approximately 1 minute and 28
Seconds every time. It only does this when the engine is cold. I have owned many Volvo in the past with the B230
engine and have never encountered this sound before.
This engine has 236k on it and was a one owner with maintenance done by an independent Volvo specialist
for most of the past 30 years.
The wagon seems to perform quite well after warm up and gets excellent gas mileage.
Anybody know what is making this noise?

[matthew1]

Sounds like a worn bearing. I'd go waaaay out on a limb and theorize that while the engine is warming the tolerances of the component making the sound are changing due to metal thermal expansion.

[Dldowns]

Thank you for the input. I have timed the noise now every day as it only does it when the engine is cold. It is exactly 1 minute and 28 seconds when the noise abruptly stops. The only thing that I can think of that functions this way would be the thermostat. Could the noise be related to the water pump reacting to a closed
thermostat? I have listened to the pump but I don't hear
anything unusual. I live in Reno NV and have driven back and forth to Sacramento a lot so far this summer, and the wagon just chugs up the summit quite well, never doing anything to suggest a problem with engine internals.
I am going to investigate an exhaust leak, but doubt that is the noise.

[matthew1]

Could the noise be related to the water pump reacting to a closed
thermostat?

Good thinking. Possible it's the culprit.

I have listened to the pump but I don't hear
anything unusual.

With a stethoscope?

[Dldowns]

I don't have a stethoscope but I will try some of the
"Tricks" for listening for noises. Can't wait for tomorrow
morning!

[Vova585]

Agree with Mathew1 on bearing. My guess is water pump vs timing belt tensioner pulley. 2 possible ways i would adress this issue depending on my day and preference.
1: Agressive approach: remove all 3 drive belts while engine is cold and manually turn each component to feel if any bearing/component has play. If everything feels ok-start the car without drive belts. Avoid prolong idling not to warm up defective component so it will stop making noise. Noise present-time for timing belt idler investigation. No noise-turn the engine off and put back water pump belt. Start the car. Noise is again present- water pump is defective., no noise-time to add 2nd belt for alternator and retest as above. Noise present-alternator is bad, no noise-time for 3rd belt test for ac compressor.
2. More relaxed aproach- Car is cold. Remove AC compressor belt and start for brief period. No Noise-bad ac compressor clutch bearing vs internal bearings in ac(doubt), if noise is present- remove 2nd belt and retest...

[Dldowns]

Thank you for the suggestion and instructions. I stopped by the Volvo service shop in Sacramento yesterday and played them the video of the noise. They were 99% sure the "knocking" noise was coming from the AC compressor.

[Dldowns]

UPDATE: because the 945 was originally sold in California, it has the pulse air system. The sound is
from air being pumped into the exhaust manifold ports.

[abscate]

Just found this thread. Your description is text book of injected air pump but I don’t know the 940 at all. On The v70, the pumps are vulnerable to water collection in the pump case which rusts them out, maybe check the 940 Fora to see if that applies?