probably the header tank cap. i noticed mine used to bubble and sptted that someone had tried to get a better seal with silicone.

i still have that same cap but recently changed to an 82 thermostat and once id got the air out the system it hasnt done it. my t'stat housing bolts were loose aswell wich allows air into the system and quickly pressurises.

a dab of silicone over the head bolts is an added safetly measure along with loctite. a trick from racing were lockwire is unable to be implemented due to access.

are your fans right up front or aft intercooler (between ic and rad) or behind the rad? atm my fan is in front of the ic and is nigh on useless. going to mount a slim one between rad &/i/cooler