This may sound odd ... but I personally am always trying my weird training ideas. Some of them actually work! Lol! XD
This is what I would try......................
Could you place a "High Value" treat up on the shelf of the unit itself?
Day 1 ... With the unit turned "OFF" ... show Blue the treat sitting on top of the unit and make sure he sees it ... give it to him. Praise him. Do this a few times throughout the day. Do this for as many days as you feel is needed that Blue seems comfortable with this.
Session 2 ... with the unit turned "OFF"... pick Blue up and show him the treat with him at a closer distance to the unit ... give him the treat. Repeat throughout the day. If he seems scared ... do not force him near the unit ... stay further away or go back to day 1 ... until you feel he is ready to go near the unit.
Session 3 ... show Blue the treat with the unit "ON" ... but do "NOT" pick Blue up ... just take the treat off the unit and give it to him. Repeat throughout the day. I would not ... however make him go near the unit when it is "ON" ... ever.
If you see that he is feeling comfortable with it .... maybe he will see the AC unit as a good thing ... and think the AC unit gives out treats?
Might be worth a shot ... like you say ... you can't control the volume.
Just space out the days however you feel is necessary ... You may need a couple days with each training session .... if it even works. :/ Just remember to not force him into anything ... praise him and treat him big time ..... he has to make up his own mind that the contraption is harmless.

If you need a calming aid during the process ... do so.
I have been doing something similar with Abbylynn for thunderstorms ... it is working some for us. She also takes a calming aid as I am trying to condition her to this. She is more than petrified of storms. :/
EDIT: If for some reason Blue seems even more stressed by this idea ... totally stop! You do not want to make the fear worse!