Cricket noise after 20 miles or so


Fourtrak 1997 50,000 miles has developed a noise which sounds like a cricket (as heard abroad in Spain etc). The noise seems to be wheel related and is louder with power on. The noise appears after driving about 20 miles or so. The noise starts at a low level and gradually gets louder. The noise does not continue when at stsndstill. Anyone got any ideas?