I received your PM, with the
link to the enclosure.
How can you describe this 'noise'? is it a clicking, humming, or spinning noise?
If you take off the top of the enclosure, and connect the drive inside, then power the dsrive, and attach it to your system, I want you to do this:
Place your palm, gently on the top of the drive, while it is (or should be) running.
Try to sense if it is spinning, which you should be able to lightly feel.
I use a stethoscope personally, as I am about 60% deaf, when examining drives. I can usually tell by ear exactly what the problems are with a given drive. You can buy one at most drug stores, and for a HDD, a traditional medical stethoscope is better than a mechanical stethoscope (probe).
It is possible that the drive is not compatible with the enclosure, but OWC do not list any compatibility charts: They merely state to ensure the drive is compatible, which is awfully unhelpful.
I am guessing that you do not have access to another Mac with SATA-I or SATA-II to check it otherwise, and thus, I sent you a PM with an offer to do datum recovery. Assuming I can recover files, I would make a DMG for you, and place it in a (protected) directory, for you to download.
This is a difficult diagnostics process, to do remotely, and if you do not have access to equipment to test a drive, you may want to consider having someone, if not me, analyse the drive. It is almost always possible to do recovery, but if the digital/power board is bad, I would need to replace that component.
You will need to decide how important the datum on it is to you, and to what lengths you want to venture in order to recover it.
If you have another system of any kind with SATA, even a PC, you could check the drive in that, with a variety of tools. I have systems set up (Linux and Windows) that are also capable of HFS recovery, so that is also an option for you, if you have other systems on hand.
I could repair your G5 too, but it probably wouldn't be worthwhile in terms of cost--the freight alone would be absurd--compared to buying another one.
Replacing the PSU on a G5 requires dismantling it, and it is very easy to damage the CPU modules, if you aren't extremely careful. You may wish to locate another G5, and transfer any PCI-X cards, special video cards, and RAM, assuming whatever model you acquire isn't above the specs of your deceased system. You can then keep the old one for parts.
There is a (small) chance that the power button board could be bad, and you can test that by bridging the correct connectors, essentially manually tripping the power. I'd need to verify what connectors you would want to bridge to check it, and if you have never done any electronics work, you need to be very careful, to avoid further damages, or possible injury, despite the low voltage. (One can never be too careful.)
