Question on BMW 3-Series Cam Sensor

I purchased a 2002 BMW 330xi about a month ago. When I'm driving at a steady speed on fairly level terrain while holding my foot steady/still on the accelerator, the rpm drops about 200 rpm and then back up 200 rpm. It will keep fluctuating up and down like this every few seconds as long as I don't move my foot on the accelerator and the road remains fairly level. This seems to be the most noticeable around 40 mph, but does it at other low speeds as well.

Also, between 60 & 70 mph there is a slight vibration and hum sound. Tires were balanced and four-wheel alignment done which did not make any difference.

I had it at a BMW service garage for 2 days and they could not determine what the cause is for these issues.

Thank you for your help.

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[Nick at Pelican Parts]

is this a stick or an auto? either way it sounds like the drivetrain is slipping.

- Nick