Air compressor back pressure advice (Wabco compressor)

Hello,
I have a 03 VNM D12. The compressor is building pressure with no problem. There is back pressure until the air dryer spits. Once air dryer spits, the back pressure stops. Once pressure builds to 120psi back pressure begins until air dryer spits, and the process repeats itself. Is it fine for a while as long as it's building pressure? I have a video attached to show the problem. Any input would be greatly appreciated.

Thanks in advance,
Tim

 

The website will not allow me to upload an mp4 video which shows the pressure build and the backpressure noise associated with compressor.

 

Pics of compressor