Knocking in Heater core return line

[midwestLTD]

Any one ever heard this noise?
Heater core return line knocks w/ a pressure surge all the way to the thermostat!! At least I think it a pressure surge. New heater core about 4 yrs old, thermostat less than a year.. Any ideas???

[86VickyLX]

I've heard of like a sloshing noise, but not a knocking noise.

[sly]

I've heard gurgling... but not knocking.

[mcninetyone]

I've heard that knock.......not long followed by a heater core failure.

[gadget73]

I think your heater core timing is overly advanced, or you are running low octane coolant.

[tjc78]

I think your heater core timing is overly advanced, or you are running low octane coolant.