Strange sound coming from passenger side under hood

Hello

I have a 2014 TRD Offroad, Prerunner 2wd. I started hearing something under the hood like a pump. At first not very noticeable, however in the last 3K miles, it's comes on more frequently. Took it to the dealer they hear it but don't know what it is, they have ruled out the air pump. It will not do it sitting idle, it starts at about 25 mph. Very annoying since it is a new truck. The only change was I started using synthetic oil, and installed a TRD oil filter. Kind of sounds like the A/C compressor, but coming from the right passenger side. Has any one had this problem?

I have friends that have heard the same thing on their trucks, but have just learned to ignore it.