Hi guys I had an underfloor heating system installed it's a mat based system ontop of concrete sub base and water pipes . Then 2 part self levelling compound on top Been down 6 weeks about 12 mm on top of matting Looks solid but now a few hairline cracks appeared and every few hours I can hear a cracking noise I'm guessing this is from the self levelling compound Inna few areas it does sound hollow if knocked on so we dug an area up to see if it was bonded to the matting it seemed to be bonded well. So any ideas in terms of should we be ok if put a decoupling membrane on top to tile it Will the hairline cracks and noises stop over time ? Many thanks for help Doc James