A few days ago I popped into the van parked in the drive. Immediately I could hear a low volume high pitch screech, one of those sounds that is difficult to identify exactly where it's coming from. I checked the normal alarming units..fire alarm, van alarm, carbon monoxide alarm, vehicle immobilizer alarm. I was sure it was coming from the low level outside temperature unit, even took out the batteries but no, it kept going. Les came out and between us we worked out it was coming from the cab area but we could not pinpoint it's location despite crawling about the floor and rummaging through all the stuff on the overcab. Eventually I started the engine and the alarm stopped and did not come on again when I turned the engine off........That left me even more baffled.... Next day, all was quiet. Yesterday I went in and it was going again, louder this time. Another hour searching and nothing found. Again I started the engine and again it stopped. Right over to you lot. I now know what it was but have you any ideas?