I have recently upgraded from a Canon Digital Rebel XT to a Canon 50d. I am extremely surprised by how loud my XT's mirror now seems in comparison. I'm pretty sure that it's not just that the 50d is quieter, the XT seems quite loud. Furthermore, it has a much "sharper" sound. Does anyone know why it is louder and if there is a way to quiet it?
I've had a few Canon DSLRs, one being the Canon Rebel XT. It does have a distinctive sound, one that I could pick out of a blind lineup against other Canon DSLRs.
The biggest reason that it is louder is that it was a very early model in the entry level digital SLR camera arena. The Canon Digital Rebel was the only other predecessor in the line. Therefore Canon simply didn't have much experience in this mostly plastic body production. Along the same lines, adding in the additional hardware necessary to keep it quiet was likely low on the list of important features in such an early low end model.
Since then, many models have come out with special silent shooting modes. These really take the sound dampening to a new level and are a great reason to upgrade if you need this feature. I have found the modes very useful when in a Church or similar situation.
How can you quiet it? Really you have very few options. The one I will suggest is to use mirror lockup if the Rebel XT has it(sorry I don't remember off hand). This will keep the mirror up but obstruct your view, not necessarily reducing the noise, but letting you control the noise at least.
See these other questions for more detail on this topic:
The sound is the sound it makes. Newer and higher-end bodies often have more dampening to dull the sound of the mirror-slap. This is not the case for every model because there are trade-offs. Sony claimed, when they still made DSLRs, that mirror-dampening introduced complex vibrations which made the stabilization system (which was built-in) less efficient.
Some newer models, even entry-level ones offer a special quiet mode which slows down the mirror and delays its return. This has several negative consequences but make it very quiet when required. It is not something to constantly use though.