A few minutes after ourTruma 6002 starts to actually output heat a rough intermittent roaring sound starts, only stopped by turning down temp. The heater is in a side locker, while it is making the sound the convoluted black plastic outer ducts visibly vibrates, holding the duct quietens it a little but not completely. The heater has worked well for the last couple of years with no problems, nothing has been changed in that time. The exhaust/intake is through the roof, the outlet does not seem damaged or blocked.