1993 Chevy Silverado Acceleration

I have a 1993 Chevy Silverado with 80,200 miles on it. I've noticed lately that when I accelerate everything sounds normal. When I get to about 30 or 40 miles per hour I hear like a gargling sound. When I get my foot off the gas pedal it goes away. When I press it again I hear the little gargling sound again. This is only when I'm at about 30 mph and faster. Thank you for your help.

I already added a gas additive from STP to see if it goes away but nothing.