Its another issue we come across on a regular basis with spas. The first issue is a spa pump that hums and sounds like its working, but without any water actually coming through the water jets. The other issue is a pump "hum" becoming increasingly loud and to the point where it can be heard by neighbours.
The first issue, where a pump can be heard but not felt through the jets, is caused by an air lock in the hot tub. This usually comes about after a drain & re-fill, which allows little air pockets to emerge in the pipework. These air pockets then prevent the pump from pushing water through and can cause it to burn out. One method to clear the air is to turn the pump on and off repeatedly, slowly pressuring the air out through the jets, whilst this method is effective sometimes, it can also harm the motor, so we recommend not continually doing this if it didn't work the first few times. The second method to clear the air pockets, is to release air, either on a pump air release valve or by simply unscrewing the pump union until all the air is out and water starts to come out. These methods will clear airlocks and you will see air bubbles coming out of the water jets as it is pushed out.
The second issue, an increasing pump humming noise over time is less easy to diagnose and resolve, but some common issues have been found. The increasing loudness of a spa pump usually develops over a period of time, as the spa has settled in its position, the vibration of the pump motor gradually moves and loosens certain parts of the spa. The best way to diagnose a vibration issue is to remove all panels and listen carefully to the noise source, usually it can be certain screws, bolts or seals, which have slowly loosened over the hot tubs lifetime. Most spas have vibration pads underneath each pump to help reduce this at first, but again, these can become loose to the pump and be causing the base to vibrate. The best way to resolve this issue is to feel for vibration and tighten any screws or bolts, which you can see are moving. This humming noise can sometimes be due to an ineffective spa pump motor which is damaged, so ensure the condition of the pump is OK.
Happy Hot Tubbing !