2002 subaru pfft sound

I have a 2002 Subaru Imprezza 2.5L engine(non turbo) when cruising at 65 to 70 mph every 18 seconds(approximately) I feel a very slight hesitation in the engine. I also hear a slight pfft(like a pressure release) from the left side of the engine compartment. When the car is in park and I crank it up to 2700 (this equals 65 mph) there is nothing. It is so slight that if the road surface is rough you cannot hear or feel, but on a very smooth road surface it very noticeable.

Any ideas??? All I’ve done is confuse one Subaru mechanic.

The sound you are hearing may be due to a vacuum leak somewhere.