Vaccum Sound When I Turn On The X

[Blade69]

Every now and then, when I start up my X, I hear a little motor or vacuum sound coming from the rear compartment. Since I have the air suspension, I assumed it was coming from there. But why all of sudden is it doing it? It's only occasional. I can't pinpoint when it does or doesn't do it. Outside temp has no affect. I thought that since it was getting cooler and air is thinner that it was replacing air in the chambers. My vehicle doesn't raise or lower when I'm sitting there listening and looking while it's running. It's just weird.

I had some minor suspension work done (not to the air suspension though) and I can't recall hearing it before I had that work done.

Any help would be greatly appreciated.