Knocking behind firewall on right side '96 E36

[PGS2016]

Trying to identify a knocking behind the firewall on the right side. It starts when I start the engine and stops after about five minutes. Sounds like a valve trying to open. Any suggestions?

[Eric93se]

Well if it were cold outside then that could be your SAP (secondary air pump), but it's warm out and it shouldn't be running unless one of your temp sensors is bad.

[PGS2016]

Thanks Eric93se, it's warm out, in the 80's, but I'll check it out any way.

[shogun]

Use a mechanics stethoscope, much easier to find the source. https://toolguyd.com/mechanics-stethoscopes/
I once had the problem, that the vacuum pipe which goes to the flap for emission control was brittle and the valve always was flipping, not # 2, the other side http://bmwfans.info/parts-catalog/E3...trol_air_pump/
here you can see it, # 10 http://bmwfans.info/parts-catalog/E3...acuum_control/
replaced #10 with a silicone hose and the noise was gone

[Eric93se]

Car usually has 3 temp sensor, one for the climate control, one for the computer and one for the dash. Which one is used for the computer to turn on the SAP. I guess it could be the block sensor for the computer since the sap only comes on during started. You have a 328i and there is only two temps sensors, the one on the block is a combination sensor, there's a chance its bad. If it is bad it can also mess up your mpg, it's cheap I would replace it. Not too easy to get at though, would be easier if the intake was off the engine.

[PGS2016]

That, I'll check it out.

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Thanks, I'll check it out.

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Thanks