The sound is indeed the duovalve. It cycles at a rate depending on the need for heating in the heater core. With the A/C on, the duovalve will still be required to help maintain the core temperature. In the spring and fall, when minimal heating is required in the cabin, you will hear the valve cycling more. Both of my 202s do it. The gurgling is the water being let through the heater core a small slug at a time as the valve cycles. I recall posting a lengthy reply on this subject to someone on club202.com.

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'96 C280 (gone)
'97 C36
'05 C230k