Home theatre /philips hts 3440. when i first turn it on it works fine! Then after aPPROX 10-15

Question

when i first turn it on it works fine! Then after aPPROX 10-15 MINS it makes a buzzing sound and the sound goes off and subwoofer fan comes on, and it stays this way indefinately! Any reason why? Is this a problem with this unit? Other than that this syatem actually sounds great! Thanks for any information you may have!