When I turn the machine on it makes a buzzing noise, (not loud but it might be like an alarm).

The machine needs stripping down as something it likely worn. The gearbox is also packed with grease and may need re-packing. They do need periodic servicing. Does it make the same noise with the bowl removed? Sometimes the noise will be attachments hitting the bowl. There is an adjustment screw underneath to see clearance.

https://www.google.com/search?q=SMEG+Stand+Mixer+makes+loud+clanging+noise

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call in an electrician as it may be the start/run capacitor for the motor ( indicated by motor humming but not running)
the gearing will make it very difficult to turn the blades by hand as the motor will be spinning at either 1400 rpms or 2400 rpms down to around 50 rpms for the blades

It is more than likely in the planetary or the fuse gear inside the gear box. Try it once you have the planetary off before you go looking inside the gear box. Easiest is planetary problem but far far more likely is the fuse gear.

Could be a problem with the start windings.or start switch.maybe a capacitor .on the older models the start switch sticks just needs freed up and contacts on start switch cleaned.

thebsjunk - I don't know if you've canned the old mixer. If you've bought a new KA, congratulations to Kitchenaid. If you haven't, there's still a possiblilty that you might be able to fix it yourself. (For the price of KA mixers you'd think they's last a 'lifetime', but alas, KA quality is sorely lacking from their current products...but I editorialize.)
It's quite possilbe that you've had a gear tooth break in the transmission, which would lock up and prevent the planetary gear (agitator) from rotating in mixing bowl. This is what happend to my wife's 6 Quart Pro KA mixer. Since it was out of warranty, I decided to troubleshoot myself. First I downloaded the Parts List manual from the KA website so I knew what to expect. Then I easily disassemble the mixer to find that a bevel gear tooth had broken. After ordering the correct parts, and reassembling, it works better than when we first upacked it. The loud whining is absent...
If the motors gone bad, you can still replace that, too. Fixing your mixer yourself is much easier than shipping it off to a repair center, and you have the gratification of doing it yourself.
I hope this helps! Howard, Burke, VA

You may have a wire not fully seated in your plug, or the worm gear you've just replaced is bound by something ( a gear tooth?) or wrong worm gear. Check to see if all the gears are seated properly. You should be able to turn the agitator by hand. If not, something is jammed in the gears. The other possibility is the windings in the armature are bad.

Hi. These mixers normaly make a lot of noise when in operation. Go to the shop that sold it to you, or any place that has these units on display and ask them to turn on the mixer for a demonstration. If their mixer does not make the same noise, then contact Kitchenaid Customer Service and ask for a replacement under warranty. If this solution assists you, please rate it. Good luck! ricardok45

this is common for any device that has a motor,either the gears are grinding or the motors contact brushes need replacing, it will have to be looked at to determine the exact cause

the company jumped right in and helped me out. impressive.

If it's metallic sound you are loosing teeth on a gear or two.  This is caused by lack of lubricant or simply fatigue of the material of the gears.  Don't use it, you can get metal flakes in your food.

Take it in to a repair shop and have it fixed.  The money spent is well worth it because if done correctly will last another 12 years.