I just stick my head down by the grill and listen for the sound. You can hear the heater as it warms up. takes about 3 seconds after you plug it in. I would describe it as a roar, but really faint. I have had to replace the plug end of my heater before (the part that sticks out from the grill).

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andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles