I've been seeing very low reported downlink SNR on XM radios that have been elevated. This is the SNR reported in the SM dashboard. SNR reported on the AP is typically very different. Downlink MCS rates tend to be all over the place though, so the noise situation could in fact be as bad as the SM reports it to be. Without the ability to remote-run a spectrum analysis though it may have to remain a mystery.
Consider the following:
On the AP for the same SM: