Might be talking shit but sounds like air lock to me
from an earlier post by ardsaliah
The easiest way I've found of bleeding the coolant system is to let the expansion tank do the hard work. You have to fill the expansion tank about 3/4 full of coolant, and when the system warms up air gets vented into the expansion tank as it's pushed out the rad and it bubbles off, and when the system cools down it sucks coolant back into the system from the expansion tank. With an 'overfull' tank it always has fluid to suck back in. About 2 or 3 cycles like this and your airlocks will be gone, but remember to have the heater on full heat so that the heater matrix is actually in the loop, and you may get some coolant sprayed out of the expansion tank, which is normal for this procedure and nothing to worry about.
Rich