What is this noise?

Here's hoping someone else who has experienced this can lend an ear! The car is a 2002 C4S. It doesn't happen every trip out, but sometimes happens multiple times per trip. I think it only happens when starting from a complete stop, I don't think I've experienced it from a "rolling stop". It also doesn't happen immediately when the wheels start to roll; its delayed a bit from starting. It usually starts couple seconds after the wheels start rolling, foot on the throttle, usually still in first gear. It doesn't seem to matter if I'm turning or going straight. It doesn't seem to be connected to the engine revs/throttle, or if I'm gentle off the line, or more aggressive. I never hear it once I'm at speed, or when coming to a stop. Separately, I do get a bit of brake squeak coming to stops now and then as well but that's not this. I never get this sound when stopping. I left my phone recording on the seat beside me the other day and finally caught it. The attached recording captures two gentle start / stops; the first 10 seconds is one start, first gear/second gear, and then coming to a stop at ~10 seconds; then a gentle start start again at around 12 seconds, with the noise at the 14 second mark. My best guess is something to do with the brakes, but I've had them looked at and they didn't see anything obviously wrong. And of course it hasn't done it yet when its at the shop. Thanks in advance for any suggestions.

I don't think it has to do with the wheel brakes. Perhaps SAI pump or the master vac?

My 2002 C2 Cab makes exactly this same sound under the circumstances you describe, and has for some time. It doesn�t seem to affect anything. Of course, it�s so random that it never happens when I take it to my indy for service. I�m sure hoping somebody pipes up with �I know exactly what that is. Seen it and addressed it.�

The sound reminds me of the RMS "squall " for a vibration caused by excess crankcase vacuum sucking the seal too tightly to the crankshaft ... Test the AOS immediately with a manometer !!! .....( only takes 30 seconds) The root cause is not the RMS, but rather the AOS is causing the RMS to vibrate/squall .. I hope I am wrong for your sake, but if I am right, install the UAOS and not an OEM AOS .. Note : other seals like axle seals and frnt drive shaft seals on a C4 can also make the same noise although a different root cause..

Thanks for the suggestions. The shop noted the AOS should be done next time I bring it in for service. So I'm booking that now since its due anyway. If it resolves the noise as part of that, even better. They're ordering in the UAOS. I'll mention to look at the the seals you suggested as well. Thanks again!