Loose valve might be worth checking, but i'm betting on it being a slight bead seal leak. JimFreeh's method is the best check, adding a little dish soap to the water will make it even more visible (and fun for kids!!). While you have the soapy water out, brush a little onto the valve stem and valve.

Good luck.

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Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)