I'd like you to picture the following situation - it's about 11 O'clock at night, it's very dark and very quite. You see a man on his drive, on his knees scrabbling around placing his ear at different points on the bumpers of his car, front and back - he seems deep in thought. You approach the man and ask him if he's "OK?" he responds "Someone on the Internet told me to do it". I'll leave the rest to your imagination - needless to say my neighbour thinks I've thrown a cog.

Having said the above. . . You are spot on.. . ! - thank you for that gem of information. I have a clicking (4hz) from each sensor although some produce a "cleaner" 4Hz. I will this evening check all the electrical connections and re-listen. . I'll then replace the ones not producing a "clean" 4Hz.

Thank you once again all, I'll keep you informed (as long as my neighbour does not have me committed).

Cheers all.