My mom has a '99(I think) Town & Country, which is essentially the same vehicle, and hers started to hiss and a few weeks later she complained of it not getting cold enough. I put my gauges on it and I think I remember adding at least 2 twelve ounce cans in there and all was good with no more hissing. I'd check the charge.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo