There is almost certainly too much resistance somewhere in the machine's air flow. It could be that the filter (if fitted) needs a clean (or even replacing) or the bag/dust reservoir are full. If all ok, check that none of the tubes are blocked. (The air is being forced out of the relief channel due to resistance in the normal 'through flow' of air.)
Answers & Comments
Probably a problem with the filter. I suggest you take it out of the machine and thoroughly clean it before replacing back.
There is almost certainly too much resistance somewhere in the machine's air flow. It could be that the filter (if fitted) needs a clean (or even replacing) or the bag/dust reservoir are full. If all ok, check that none of the tubes are blocked. (The air is being forced out of the relief channel due to resistance in the normal 'through flow' of air.)