Just read your message a second time. Is the sound following the release of the PTT button? If so, it is indeed the Roger Beep (aka talk confirmation tone). If holding in the minus button while turning the radio on does not turn it off, there might be something wrong with the radio, such as a one of the other buttons being slighty stuck. Or you have your keypad locked. To unlock the keypad hold the menu button in for 3 seconds until the lock icon disappears.
Another thing which will make a noise during or after transmitting is the low battery beep. You might check your batteries. Old, bad, poorly charged or defective batteries can cause this.
fyi: Some users might think the Call Tones are the Roger Beep.
Lastly, there is a PTT Timeout beep which will occur if you hold the PTT in for more than 60 sec while talking.
Hope one of these items is of help to you.
http://www.giantintl.com/ContentManage/manuals/Moto/T65xx.pdf
Best regards.