also, if the car has a coolant leak or if you recently added coolant you may just have air trapped in the system. I had a pretty bad coolant leak my 88 300E and since I was always adding coolant whenever I had the heat on it would gurgle. Once I got the leak fixed the gurgle stopped.

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E30 B25
126.036
123.130 with 616-Frankenbenz (sold)
124.030-Greyhound (sold)
124.030-Schnauzer (sold)

DERP DERP