Macbook Pro running with lid closed

hi guys,

i am having this problem from the very beginning: my macbook pro 15" non retina is running with the lid closed. when i close the lid it goes to sleep but when i come back a few hours later the fans are running audibly and the mbp is warm, sometimes hot.

this did not happen to my previous mbp - so what is the problem here?

thank you,

j.