What was found
SJU FLT 1035 EN ROUTE SJU TO CLT. ABOUT 23 MINUTES INTO FLIGHT, FLIGHT ATTENDANT IN THE BACK OF THE A/C REPORTED A BURNING RUBBER SMELL. THE FLIGHT CREW EVALUATED THE SITUATION FOR ABOUT 3 MINUTES. THE BURNING RUBBER SMELL GOT STONGER. THE FLIGHT RETURNED TO FIELD SJU AND LANDED WITHOUT FURTHER INCIDENT. INSPECTED A/C. RAN BOTH ENGINES AT POWER AND APU ALL CHECKS OK. REPLACED HEPA FILTERS, FOUND CLOGGED.