The heat in my blazer when warm blows hot like it is supposed to but it turns to cool air until there is a wierd sound in the dash like a suction sound as if a vaccume line is closing then it goes back to heat then back to cold. ??????

 

Sure it aint more a 'gurgle'?
How does your engine temp gauge act? Does it stay steady at just under 200?

Low coolant/air in your engine cooling system. Open radiator in morning when cool. Should be all the way to the cap.
Check your engine oil dipstick and oil filler cap make sure no milky/chocolaty looking deposits on them

 

No coolants good checked it several times it like the climate control is always clicking on but there is a definate noise in the dash in drivers side before it swithes to heat like a thermostat is triggeeing the air .

Temps are good too.

 

sounds like a vac hose got loose in the dash

like the bend door hose
what are you driving ?

 

99 blazer Lt

 

id listen for the hissing sound again and see where its leaking from