Attack on MIDI track...

Hi all,
I’m trying to get MIDI file creation into my head and I’ve got to the point where I can create controller tracks in the list editor which creates a ‘header’ on the actual MIDI track. The problem I’m having is that one of the tracks of the MIDI file are strings. GM sound No.49 (String Ensemble 1). When I add the controller info which is just the program name, and the volume level, the strings have a slow attack on them. If I delete the header, or change bank number whilst it’s playing, the strings return to normal. But if I leave the header, the attack remains. I haven’t added any attack info, so I can’t understand why this is happening.
Any thoughts?
Thanks, Steve.