Valve Tapping noise

[fema]

My ML320 has suddenly developed what sounds like the old valve tap you used to hear on old american cars - is this unusual? With foot on the gas and accelerating the noise is clear, foot off the gas or lightly on to hold speed you don't hear it.

I looked the vehicle over last night and found nothing loose or vibrating; thought it might be a belt slapping or loose mudflap, but that's not the case.

Thoughts please, thanks.