Mixer infected by foul demons makes bad noises

Hi, JSTEWART431, are you sure it’s the motor or could it be the transmission gears where the noise is coming from? Are you sure it's the motor or could it be the transmission?
It 'sounds' like you've got some transmission gear issues. It could be as simple as through normal use the gears have shifted slightly in the transmission and the gear teeth are making different contact, hence the grinding noise. The higher the speed, the louder, higher pitched sound you'll get. Alas, the noise may also be indicative of a potential gear failure, similar to my wife’s mixer where the mixer’s increasing noisiness finally ended with a broken tooth on the bevel gear and the mixer 'froze'.
If you are DITY type person, feel adventurous, and your mixer is not under warranty you can troubleshoot it yourself. Be sure you have the Parts List manual on hand. If you're lacking one, you can download a PDF version from the KA website: http://www.kitchenaid.com/flash.cmd?/#/page/home. Click on Customer Care>Locate Manuals & Guides> Enter the 6 character model number, in your case use KSM90P, and select the first file listed under Parts List (16) and then check to verify this matches your mixer model.
Turn to pages 4 & 5, titled: Case, Gearing And Planetary Unit for the disassembly and reassembly of your mixer, along with all the parts. Locate Illus. No. 36, Worm Gear Bracket & Gear, including Illus. No. 7, Worm Gear. I suspect this is the weak link in your mixer’s transmission and causing the problems. After clearing all the grease away, check to see if the gear is just loose, exceedingly worn or broken. If it’s worn or broken you can order a replacement part and install it yourself. While you’ve got your mixer this far apart you can check for any other unseemly wear and tear on the on the gears and other components. Besides a replacement gear, and grease you might consider replacing the gasket, too. You should use a food-grade grease that you can order online (although I’ve seen automotive bearing grease used too – your call).
The next possibility to check would be with the motor itself, but electric motors are pretty simple mechanisms and generally don't fail. Using the parts list, you can see what needs to be inspected and determine if any motor parts need replacement. (Generally only brushes wear out, but I don’t think that would cause your grinding sound.)
Here are five potential vendors in no particular order: RepairClinic.com; mendingshed.com; searspartsdirect.com; partstore.com; and Grainger.com. You should search for ‘food grade grease’. Prices can vary widely between vendors, so do your comparison shopping.
It can be messy but quite gratifying to diagnose and repair your mixer yourself.
Finally, here’s a link to an excellent site for disassembling/fixing your KA Mixer: http://www.neilcrockett.com/mixer.htm and another link to a site that details the disassembly and replacement of mixer’s grease, which might be helpful, too. http://www.foodonthefood.com/food_on_the_food/2009/04/how-to-repair-a-kitchenaid-mixer-yourself.html
If you feel troubleshooting and repairing is beyond your capability, then I would seek out a local small appliance repair shop in your area. KA Customer Care can give you names of local KA authorized repair shops in your area, but you can also do an online search to supplement what KA may provide you. Call to explain your circumstances and find out their hourly rate and get an estimate of what they think it would cost to repair your mixer. Also, always obtain three or more estimates – do some comparison shopping.
Good luck. I hope this helps. Howard, Burke, VA
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