"Whirring" noise coming from passenger's side?

I installed Eibach 25mm (1") hubcentric wheel spacers on my '98GT a couple of months ago, and have been experiencing a nasty whirring/howling type noise coming from the rear passenger's side of the car only when making sharp left or right turns. This noise is only apparent after a long drive, let's say 25+ miles, and isn't noticeable at all on short commutes. The problem began about 2-3 weeks after the install of the spacers, so I don't know if they were the cause of the problem, or if they merely amplified an existing problem. We have pulled the rim 3 times to look for any visible rubbing or damage, but nothing is obvious! No signs of rubbing, loose parts, or unusual wear on the tires. The spacers were installed using liberal amounts of loc-tite, and we also re-torqued them for good measure. Help!

Here is the suspension set-up on the car:
'98 GT, 73K miles
'01 Bullitt springs
Bullitt (Tokico) shocks & struts
Eibach 25mm Pro-spacers (aluminum spacers)
17" premium package 17"x8" rims (10 spoke)

Dusstbuster said:

check your pistons on the rear calipers. I was getting a humming like noise almost after putting new pads on and eventually took the calipers off. Had to heat them up with a torch and pull the pistons out because they were so rusted in place. If thats your problem just heat them up, pull them out (watch out when you pull, hot oil can burst out i found that out the hard way ) and then sand it down, relube it, and put it back together. Havent had a problem since other than now my front wheels are making some noise that i cant figure out no matter what i sand/lube.

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Already had the same problem after putting new rear pads on the car. In fact, I installed the new pads about 2 weeks after the spacers. The piston was completely stuck, and we did sand/lube it. I assumed that more brake dust would be apparent on the rims if the caliper was sticking, again, but that rim has stayed just as clean as the others. I guess we'll be pulling the caliper to check. Thanks! Any other suggestions?