please advise a silent air pump

I found doing 2 things would help dramatically. 1. Hang your air pump by a bungee or string( easy and works great) or create a box to hang the pump in( works better but more time consuming). Its best if its hanging on a bungee or elastics to reduce vibration (or create a platform that it gets firmly attached to and have that hanging. the section the tubes and power cord come out should be foam to avoid vibration. Don't forget that the pump needs to have fresh air come into the box so don't seal it completely. 2. This one was huge. have your air stones not touch the bucket's bottom or edges . When they touch the bucket they vibrate and make the same loud noise. I have the tube holding the stone 1 cm above the bottom of the bucket and it drastically reduced the noise.

Hydrofarm AAPA15L Active Aqua, 6W, 15 L/min Air Pump 4 Outlets. This one worked beautifully and was very quiet. This would only be if you have 4 separate buckets or 4 intakes in a large res. Otherwise, I would look for the same brand, but a different number of outlets. Leaving any of the outlets inactive will greatly increase the noise.