Juan Gabriel -- Heart Problems Behind Sudden Death

Juan Gabriel had a history of heart issues which played a role in his unexpected death ... TMZ has learned.

Law enforcement sources tell us the Latin singing sensation was also battling diabetes. We're told the L.A. County Coroner is calling it natural causes -- Gabriel's heart and diabetes issues are listed as contributing factors.

We're told there was no foul play, and no drugs found at the Santa Monica residence where his body was found Sunday morning. Gabriel's last performance was Friday at The Forum in L.A.

He was 66.