1998 Dodge Ram HISSING NOISE

MY TRUCK MAKES A STRANGE HISSING NOISE WHEN YOU FIRST STEP ON THE GAS. AS IT COMES BACK TO IDLE ABOUT 1000 -RPM IT STOPS LIKE AN AIR LEAK SHUTS OFF. IVE RESEARCHED ON LINE.
AND FOUND IT COULD BE THE PURGE SOLENOID FOR THE EMISSION STUFF. SOMTHING ABOUT IT NOT BEING IN THE RUBBER GROMET AND BRAKET PROPERLY AND SHORTING OUT? I COULD NOT FIND IT. BUT I WILL LOOK AGAIN. ALSO WHEN YOUR GOING DOWN THE HIGHWAY AND STEP ON THE GAS THE AIR COND. SWITCHS FROM DASH TO DEFROST THEN WILL RETURN, IS THERE A COMMON VACUUM LEAK SOMWHERE THAT HAPPENS TO THESE TRUCKS WHEN THEY GET OLDER? I READ A BUNCH OF POSTS ON SOME OTHER SITE WHERE OTHER PEOPLE HAD THE SAME PROBLEM.
THANKS GARY