what is the quietest air pump for my ET-2 ?

Even pumps marketed as whisper quiet typically have >30db noise. I don't know what pressure/flow rate the ET arm requires, but but if you are getting by with a low pressure pump, there is no reason to add the complexity, condensation issues, and expense of compressors. You might try the pump in an insulated & vented wood box. I cut noise of my 30db Rena 400 air pump in half by encasing the pump in a carapace of soft modeling clay, and placing pump and flow tank inside a plywood box insulated with several layers of cut-up acoustic ceiling tiles.

I used a rena pump (aquarium pump) to float my platter on my maplenoll. Its a pretty low pressure unit. it is however very quiet and if you can get away with the low pressure, is a great unit. Now that I went with the large platter 90 lbs, i need more pressure than the rena could generate. it develops about 4 to 6 psig.

I now received the Junair. This is a nearly noiseless compressor, you only hear air breathing... really! I do use in my listening room driving the Eminent Technology 2 and 2,5. You see a pic on my system site.

I also found two other solutions, one is an Ekon Air system which is a big compressor coming with a noise dampening box and the other one is a Japanese High-Blow fish tank pump also using the big dampening box and providing continous streaming flow.