Truck was in for tune-up and coolant system flush & service, Now have started hearing the sound of water inside the cap and truck has been over-heating. Does this sound like a problem with the heater core?

You need to bleed the air out of the system. Bleed screw is small and painted blue. Start truck and slowly turn bleed screw counter clockwise. Watch for steady stream of water. It will spit air then after a minute or two you will have steady stream of water. After you get steady stream of water then tighten screw and you should be good to go

Try changing your radiator cap. Sounds like it's not holding pressure like it should

There are several methods for flushing the cooling system on ALL engine cooling systems. Time honored is drain the radiator (drain ****). Fill with fresh water and run engine to operating temperature until cooling fans turn on. Drain cooling system again. Refill with 50-50 coolant. Another method is purchasing a "T" vlave (coolant flush/service kit) from Autozone, PepBoys, NAPA, O'Reilys, CarQuest, etc.). This "T" valve is installed in one of the heater hoses & attached to a garden hose. Remove the radiator cap & turn on the water supply to the garden hose. Let water run until flush water is 'clear'. Remove garden hose from T valve & refill radiator with pure coolant until the T valve drainage turns green/orange. Install T valve cap... DONE!

Hello, when the last time you have had your truck coolant system serviced? The anit-freeze needs to be reomove the coolant system flushed out (cleaned) Also, maybe a sticky themrastat and it will need to be replace along with a new radaitor cap.

. Clean the radiator inside and out---flush the inside with flush cleaner especially designed for auto coolant systems.
Flush the engine block this can be done with the radiator at the same time.
Replace the thermastat
replace the radiator cap with the correct pressure cap. GB...stewbison

I would flush the heater core out:

open the hood and look on the firewall. There should be 2 rubber hoses going into the firewall and into the truck. If factory they will have spring clamps just like on the radiator hose. Remove both lines. Then get a hose and put it in one line and run the water until it comes out clear. Then do the same thing with the other side. Do this a couple of times. Then reconnect the 2 hoses and check the level of the radiator. Then start the car with the radiator cap off and wait a while for the thermostat to open and have the heater on so it will cycle through the whole system. Then top off as needed. This should fix your problem Tim

If there are no leaks this is fine. The coolant contracts as it cools over night and this noise you hear is the system equalizing itself while coming under pressure from the forces of the water pump, radiator cap spring, thermostat spring and any other expansion - contraction issues involved in the cooling system components. Hope this helps ease your mind. Let me know. Thanks. PS be sure your system is filled with the proper type and ratio of coolant to water for you areas temperature ranch.

You may have air in the system. Let the engine run with the radiator cap off until the thermostat opens. Have someone then get in the truck and bring the rpm up to 1800 to 2000 rpm and hold it there for 15 to 20 seconds while you top off the system with coolant and put the radiator cap tightly back on. Then bring the rpm down to idle and road test the truck. DO NOT BRING THE RPM BACK DOWN TO IDLE UNTIL THE RADIATOR CAP HAS BEEN INSTALLED COMPLETELY. I suggest replacing the radiator cap at this time also.

Your coolant level is low, and you hear the coolant taking up the vacant areas....bubbling...due to low coolant.

Hmm sounds like you might have a leak into the cooling system such as a seeping head gasket.

Have a local garage do a leak test, and if they have the tools a coolant dye test, which will test for combustion gases in the cooling system.

This sound you hear is your cooant going to your heater core. Check your coolant level , radiator cap and thermostat. one of these mught be on its way out. If you have no problems with your heat or a/c then flush your radiator and don,t worry about anything. I hope this helps. Don,t forget to rate my answer, thanks