Strange Noise

I own a 2004 GMC Envoy SLT, 54,000+ miles, 6 cyl.
(bought it new)
For the past several weeks whenever I start it, in the morning or after work ( I assume it must be completely cooled down to happen as it doesn't happen any other time) I hear a noise that sounds like a hydraulic lift that lasts about 15-20 sec. I asked the service department clerk when I took it in for it's 54,000 mi service, and was told it was the "moisture compensator" due to the hot weather we've had here in Virginia. This is the 4th summer I have gone thru with it and this is the first time I have heard this noise. There has been no diminishing in performance, and no lights on the dash.