http://ap.help.mimosa.co/snmp-oid-reference-a5
Looking through the reference all I could find was .1.3.6.1.4.1.43356.2.1.2.9.6.1.1.25 which gave a particular Clients SNR which you could combine with .1.3.6.1.4.1.43356.2.1.2.9.6.1.1.11, which will give you the average RSSI for that client, to get the overall noise.
Sorry I am not more help, I only have a cursory knowledge of SNMP, so the actual details for how to do the intermixing I am at a loss to help you with. Hopefully someone else on the forums knows something, there are several guys who visit here and maybe a summons might help. @Chadwick and @morris have commented about SNMP a couple times before, they may be able to give you some insight.