R720 Fan Status issue

Hi there

In my R720 (BIOS 2.90, Fw 2.63.6.something) I have loud fans running all the time, it seems to be that the server thinks Fan #4 is not running and ramps up the other fans to compensate. So, I bought a new fan, installed it and things went back to a nice quiet level for a few days.. Now I get the noise again and I see from iDRAC that fan #4 (the new one) is again showing a red X, no RPM reported. When i pull the fan out to inspect, it is actually spinning fast - so I think there is an issue with the server reading the fan status? 

What could I try to help further diagnose and fix this issue?

Thanks