97 Trans Sport - rear fan setting affects temperature of front AC vents

Yesterday, on the first really hot (90F) day of the year so far, I noticed some interesting behaviour on the part of the AC system in our '97 Trans Sport w/rear air. With both front and rear blowers going, the outlet temps
front and rear were cool (cool enough, but not as cool as I remember last summer.) But, moments after shutting off the rear blower, the temperature of the air from the front vents would rise quickly to about ambient temp. On turning the rear blower back on, the temp from the front vents immediately became colder.
Some additional symptoms - starting a few months ago (in winter), when the AC would be running with the defogger in coldish temps (app. 40F), I would hear what I can best describe as a 'thrumming' sound, proportional to engine RPMs, coming from both the rear AC unit as well as from under the front dash. The sound would disappear with the AC compressor off. The compressor also sounds louder than I remember it last summer.
I've done a little surfing and found some references to a TXV which operates in conjunction with the rear blower switch. Is something going on with the TXV in my vehicle? If not, is the system simply low on refrigerant? Oil? Or is it something else?
Thanks for any advice anyone can provide.
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I would have the pressure checked. It sounds like it is about a pound low on refrigerant. You are changing the temperature of the evaporator core when the fan is on. A low system will do this. Also, the compressor will be louder when it is low. I would try to get someone to add 1- 2 ounces of oil when they add the refrigerant. If they say well we have to find the leak, no you don't. if adding a pound every 2 years is what it takes, don't let them sell you more. The front evap in that vehicle is an 8 hour job.

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Belated thanks for the good advice! Adding refrigerant worked as you suggested.

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