I had this problem when I fitted my front mount. I think it is the fact that the rad is restricted to cooling due to the decrease in air flow. I changed my head gasket which had only done 400 miles, thermo, fan virtually everything and it still done it. One day just out of curiousity I removed my front mount and put a standard cooler on and suprise suprise no more bubbling sound. Not saying it is this with yours mate but if you got a standard cooler hanging around give it a try can't hurt