رضا: Yes, I have...When I, for example, say 'better', the T is much closer to 'd' than to 't'. I think the way most Americans pronounce it is found somewhere between a clear dictionary 't' and a glottal stop. I know it's a relatively big span but I can't describe it better. As far as the rule is concerned, I don't think there is one. In my opinion, it varies from region to region and if you found a rule like this in some of the dialects, I bet it would not apply for the other ones.