What's the ambient temperature and what is your heater temp set to?

Also could you almost describe the sound as a "heart beat" (it can be a double tap. Dup dup).

If so, I have a solid guess of the source of the sound:.

Water valves for the heater cores.

You can easily confirm by open the hood and hold a gloved hand on the heater valves: just in front of where your right knee is if you have left hand drive. Close to the firewall. (that's on my 3.0; might be slightly different if v8 motor. )

Drove me nuts for months until one day I was poking though real-time data on my foxwell and had the valve status open and the left/right valves were opening/closing about every 3-5 seconds slightly offset from each other in perfect sync with the odd tapping sound.

If you have a scanner that will display real-time data that's even better way to confirm.

Your description exactly matches my experience with the heater valves though so I'm willing to put 90/95% odds.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)