Thanks. I'm using Harbeth SHL5 driven by NAC202/NAP200, regarded to be the some of the worst sounding amps by few folks in the wacky Naim community. :-)
In my correspondence with Jan Meier, I suggested it is unlikely that hair is trapped or has gone through the protective mesh of the headphones as the gaps are really tiny. Another possibility that Jan suggested is a connecting wire inside the earcup that may have contributed to the rattling sound. Anyway I have decided to keep the T1s as the rattling sound will only be heard at higher volume levels with certain tracks. Since I do not have any experience with the assembly of a headphone(don't know where to locate the screws to disassemble the T1), confidence level is low as I might mess up rather than make things right. Nevertheless, will attempt something later on. As the way it is right now, there will not be any rattling sound when listening at normal volume levels(8 to 9 o'clock on the volume dial). It is only at short-term (unrecommended) head banging sessions the rattling sound will be heard.
I have also decided to stick with the T1s and disregard the HD800s as I find the sound of the T1s to be just right for my listening preferences. The neutral, warm and full sound of the T1 is crucial in contributing to its musicality and listenability. I do not find the T1s to be excessively warm or muddled as some might have suggested. As a matter of fact I find them to sound slightly lean(or clinical) with certain music or recordings. As such, I can imagine the HD800 to sound even thinner and more analytical, something that I may not be able to live with in the long run.
On a separate matter, as good as the T1(or any other stellar headphones) gets, I still find something unnatural with headphone listening although I am now able to listen to the T1s for more than an hour in a single session(initially only for 5-10 minutes before it gets uncomfortable). To cut a long story short, I find the loudspeaker-based system to be more fulfilling and enjoyable.
Did Jan not offer to swap out your pair?
Also, you're exactly right that headphones are a different listening experience than speakers...something you may want to try if you haven't already is engaging the crossfeed on your Jazz. It's a very subtle enhancement, but if you leave it engaged a while and then turn it off, you'll probably notice when it's not there anymore. I leave mine engaged all the time cause I like the effect...just something you may want to play around with.
Glad you're sticking with the T1s....I think you'll get many years of enjoyment out of them :)