Alpine Pdx-1.600m High Pitch Whine

I have a pdx-1.600m when you power up the amp some times it has a high pitch whine and when you tap on the case with a screwdriver you can hear the tap play thru the subwoofer if you tap just right the amp clears up and you do t hear it again until you hit a bump or it gets jarred around again. I boiled it down to the internals because I removed the pdx-1.600m and hooked up another amp and the noise was not present but when I hooked the pdx-1.600m again the noise was there again. Any help would be greatly appreciated ? you in advance. Also when you turn the subsonic filter on the noise gets worse but if you turn switch to off and tap a few times as I said it goes away until its jarred around again