Where does this strange noise come from?

[matinee100]

I am so exhausted with the noise. The car is 93' california version 300E with M104 engine.

Symptoms & information:

- The noise lasts no longer than 2 seconds.

- The noise is intermittent. I cannot make the noise intentionally.

- The noise comes only after minimum of 30-minute driving. So I think the noise is heat related.

- The noise comes only when the engine idles, the transmission engaged(D/R/3/2/1), and the brake applied.

- The noise comes from the driver side of the engine room. The noise is loud in the cabin, but I have not heard the noise outside of the car because I could not duplicate the noise.

- A kind of low-pitched sound(around 100 ~ 200Hz maybe).

- The most similar sound is the noise of a pneumatic/hydraulic pump.

- A/C does not affect the noise.

- Steering does not affect the noise.

- The idling of the engine is very smooth. No vibration.

Anybody help?