Position 12 on a CE2 fusebox is also the carb manifold heater thingy on Drivers and GLs etc - Relay is numbered 1, Part number 171 906 383. It is exactly same electrical spec as Digi fuel pump relay but the connector layout is a mirror image 
and just to totally confuse things the Digi relay is part number 191 906 383. Guess which relay was going like a set of castanets on my fusebox today?
Barkstar 
I have had this problem with my mk2 Driver. I replaced the 171 906 383 relay because it was buzzing away. But, the new relay still buzzes, just as much as the old one. Obviously it wasn't the relay that was broken but the 'carb manifold heater thingy'. I have a 32/34 DMTL weber carb on the car. Is it that heat sensor that rests against the base of the carb? If this is faulty would it make the relay buzz?
& finally what part part no is that?
Any help would be much appreciated as the buzzing is just plain anoying!
JamesS
