Loud noise on first start-up?

For some time, when I turn on my computer, a really loud noise starts to come from the computer case, sometimes accompanied by some sort of rattling sound. However, there are no problems with the functionality of the PC itself when this noise is on. Then I turn off the PC and turn it on again and this always fixes the problem.

Now I'm not sure where it comes from. The only thing I know is that it probably doesn't come from the DVD drive. I did a partial dusting of the CPU fan and the graphics card fan (8800 GTX) and it sort of removed the problem for a few days but I don't know if this was just a coincidence. I checked online for typical sounds of failing hard drives and it doesn't really sound like it although there are some similarities in the sound. I have a Samsung hard drive which was bought along with the rest of the rig in April 2008 so it's not exactly at the end of its lifetime.

Any thoughts? Thanks.

I used to get this, I would find it can be harddrives not fastened down probperly, case fans clipping a wire and even the GPU. I cleaned out the heatsink on my 8800GTS 512 and reapplied it with Artic Silver and it runs cooler and quieter.

Make sure no cables are catching in any of your fans. (CPU, Case fans, PSU, GPU) Its wierd how it goes after restart.

I thought the "rattling" could be due to cables or something else catching in the fans but the fact that I more often have this loud noise and also that it ALWAYS goes away on restart complicates things. I'll still take a look inside the case on the weekend.

Any other ideas? Anyone?