68 Barracuda Convertible motor

The same thing was happening with my Dart when I got it. It was just low on fluid. I used plain old type 'A' Automatic Trans Fluid.

From my '64 Dart shop manual:
Insufficient fluid in system may cause top to raise slowly or cause noise in the pump and motor during operation. If low, check for a leak due to broken line or loose connections. Replace line or tighten connection as necessary. Fill reservoir until fluid runs out of filler hole. Use type "A" suffix "A" Automatic Transmission Fluid. After filling reservoir, raise and lower top several times to force out air that may be trapped in system. Always check fluid level when top is lowered.