Radio Volume Drops Slightly Above 10mph.

2010 Mercury Mariner radio (with Sync) lowers the volume (or maybe described as "subdues" the sound - perhaps removes treble) when vehicle goes from park/stopped to 10mph or higher - i.e. when you go from stopped to moving, at around 10mph the FM radio (didn't test with AM) output changes (softens) slightly (maybe 15%).    This does not happen when the iPod is plugged in. Sound stays consistent. So it's not the speakers. It happens on multiple FM radio stations. It is also consistent, repeatable. It does this every time you go from stopped to moving (as you approach/exceed 10mmph). It's a slight change to the output level, so you've got to listen for it and helps when sound is higher, but once you know what to listen for it's aggravating. I took it to a really good mehcanic and he thinks it's a reception problem and not a radio function problem. He describes the sound change as the radio loses treble. However, it does this for multiple radio stations with vehicle in the same location. Any ideas?