MOT leakage

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Borislav Trifonov" <bdt-at-shaw.ca> 

I assume subscribers to this list are familiar with microwave oven 
transformers, and that is why I ask my question here, even though I'm not 
constructing a tesla coil.

I'm working with plasma and I need a DC supply at about 3 kV and 400 
mA.   In order to save money I got a large MOT with 2100 VAC secondary. The 
thing was quite noisy and heating was a problem, so I rebuilt the core.  I 
grinded out the weldings and took apart the laminations.  The secondary had 
one end grounded to the core since insulation between the inner part of the 
winding and the core was minimal.  I stuffed extra insulation (mica) so I 
could safely unground that end of the winding (I'm bridge rectifying it 
with chains of 8x the usual 1N4007s into a CLCRC filter).  I put the core 
laminations back together with nail polish, as I have no shellac.

Now the transformer is very quiet, except when I put it in the chassis (3u 
format, steel, earthed) the chassis resonates very loudly.  At first I 
thought the problem was mechanical vibration transmission, but 
foam/rubber/springs didn't help, and I even made a pneumatic isolator with 
a pump -- nothing.  Turns out the hum was being induced into the chassis 
electromagnetically.  All my tests were with an unloaded secondary as the 
load has not yet been built.  The noise is so loud that I cannot leave it 
as it is.

I have two questions:
Will the induced hum get worse/better/same when the transformer is loaded?
What are my best magnetic shielding options?  Mu-metal foils are expensive, 
hard to find, and I'm guessing too thin and will saturate and be 
ineffective; a silicon steel enclosure will buzz defeating the purpose of 
shielding and present a cooling problem.  I'm going to go crazy trying to 
figure out how to get rid of the noise.