Brake booster whistle

On my 07 2wd v6 non stability control truck my front brakes would apply themselves out of nowhere, usually on highway drives and the fix was to get out and pull of the vacuum line to the booster which instantly released them. I bought a rebuilt OEM booster off Ebay which lasted about 5 years and now the same issue is happening again. My booster will pass the normal checks like you get about 3 pumps when the truck is off before it firms up and when the truck gets turned on the pedal will go down a little then come back up. I can only assume that pressure is building up in the booster and slowly applying the brakes but there is no noticeable noises.