Would a bad relay cause this?

[toolmaker]

Girlfriend has a 2004 XC90. Radio quit working a year ago. Don't know if this has anything to do with the current problem or not. Every once in a while she will go to start the car and the battery will be dead. I'll charge it and it will work fine for a couple of weeks then it will be dead again. Had it to the dealer twice now, but the problem is still there. There is a low hum coming from the dash when the key is off. I traced it to the fuse no.21 in the passenger compartment. When I pull that fuse out the noise stops. The owners manual says it is Relay extendedD1 feed:climate control system.power drivers seat, instrument panel. After I recharge the battery and hook it up and start the car, the AC and the fan come on full blast. I have checked with a meter at the battery with the car running and it is 14.25 volts. Would the relay possibly cause this? Thanks for the help. If you need any more info let me know.

[hoonk]

Every once in a while she will go to start the car and the battery will be dead.------ the AC and the fan come on full blast.

Could simply be a bad battery - how old is it?

Or it is possible a bad heater fan resistor is turning the heater fan on when the car is off and draining the battery. (Other than the afterblow period)

[toolmaker]

Battery should be good. I replaced it once last year. The last time we had it to the dealer, they said it had to have a Volvo battery, so they put a new one in a couple of months ago. How would I go about checking the heater fan resistor? Electrical problems are not my strong suit.