I actually already removed the PSU, cracked it open and tried to clean/desmellify it with some carefully applied vinegar. The only visible grossness was a very thin, light layer of gray residue on one of the inner walls, which was easy to wipe off. The vinegar worked perfectly to destink the outer plastics of the case, but it didn't do squat in the PSU. That's probably because I didn't get to every surface, since it is super dense. I definitely need to take another shot at cleaning it though. The smell fills the room after a while 
That's why i said: "I would just hold it upside down and spray in a 100% evaporating cleaner/solvent to flush the crud off." You can also poke at the crevices while wet with a small paintbrush.
Have you ever tried mounting a 120mm fan in place of the two 60mm fans? I read about someone trying that the other night but I forget where I saw it, or how well it worked for them. I feel like that would help with acoustics big time, but I'm not sure about cooling performance. That PSU gets pretty toasty.
That Frankenstein fan replacement required a bunch of custom duct fabrication and sacrifice of the optical cage (so no CD/DVD burner) for a small reduction in sound level and we never did hear how thermally effective it turned out to be.
Since you've had your MDD apart, you know how deceptively complex the airflow channels and such are. Messing with that should be attempted
only if you have another MDD full of parts to replace the ones you just might burn up.
You
can (and this is the practical solution) replace the two 60mm's with newer, quieter 60 mm fans as long as you match or at least near-match the airflow ratings. This is not that difficult. The secret of quieter 60mm fans is they use neodymium magnets that have much more flux vs. size and so the overall bulk of the motor is significantly reduced, allowing more airflow…simple. There are a zillion exotic fan makers and fans on the market that didn't exist when Apple designed and built the MDD. With all of the shit they got because of the noise, they would have absolutely used quieter fans if they had been available.
It's all a lesson in diminishing returns. I even have a "quiet box" - a very nice enclosure with a heavily insulated interior that encloses the entire computer. However, that requires
more fans to move air in and out of the box! Those require sound-deadening zig-zag air ducts to keep the noise from
those fans down. If you've ever tried build a studio or just quiet a room for studio use, you know that every time you identify and plug somewhere the outside noise is getting in, you suddenly begin to hear
another crack, crevice etc. you didn't hear before, then another and another……
My advice is: Don't think you're a thermodynamics engineer because you can get free, unproven advice on the internet - except
mine that is…
