Just to update: Our mechanic was able to replace the Volvo 3515641 transmission speed sensor in our 1997 Volvo S90 with Toyota part 89413-32010. Everything works so far. The warning light went away.

I am looking for a speed sensor (part # 3515641) for my 1997 S90. The same sensor is used on some 940/960's and the 97-98 V90. Thanks! PS Has anyone had success with a Toyota speed sensor and wiring adapter? Are there any compatible sensors from the other versions of this transmission? https://en.wi...

I am also looking to buy this sensor (part # 3515641). Please get in touch if you have one to sell.

According to Volvo, this sensor fits:

940 1991-1995
940 Turbo 1991-1992
960 1992-1997
S90 1997-1998
V90 1997-1998

The B6324S engine is naturally aspirated. This engine is used in both the 2008 XC70 3.2L and 2007 XC90 3.2L. I don't think it has secondary air injection. https://en.wikipedia.org/wiki/Volvo_SI6_engine#B6324S The rhythmic hum at idle definitely comes from the diaphragm which I will probably replace ...

Did you figure it out? Our 2008 XC70 3.2L has identical symptoms. The PCV would squeal. I replaced the PCV oil trap at 130k mi (Volvo Part # 31319642) and found that the diaphragm had torn. A few weeks later the rhythmic hum at low RPM's started. The hum is identical to your recording and comes from...