We have an older sears model 600 multifuel furace. Its 100k BTU,s. We presently burn kerosene in it. When we turn it on, it lights and seems to burn properly. The fan comes on , seems to be working properly. The only problem is when it is on we geyt a buzzing noise from it. Not sure if it fuel pump or what? Any ideas? Thank You

That buzzing sound is pretty mysterious; the only thing I can think of is making the fan, itself, is loose. Hmm, but if it's making sounds for long, either refrain from using it or call a furnace technician to get a free estimate/consultation, as you don't necessarily want to cause harm to your furnace, which would entail additional costs in the future. (In other words, if it's making a sound, there's obviously something wrong with it that won't go away with time, and could actually cause damage left unchecked and fixed.)

So, call some local furnace contractors around.

You might find the handyman site that I use frequently to be helpful for your situation, as it has a section dedicated to furnaces; there, you'll find a text-link ad that offers up to 4 free estimates from local furnace contractors (scrolling down, you'll see the text-link at around the middle of the page):

Furnaces ~ Your Handyman Zone :: Hammer it Out in the Zone ~ Home Improvement Zone...Heating Systems Category

On the above-linked page, find and click on the "Find Local Heating & Cooling Pros" link. You'll get the furnace expertise in no time.

Good luck!

The buzzing is constant, whether the fan is running or not.

Check the low voltage transformer. They make a buzz sound because the laminated plates come loose. There is no harm caused by the buzz only noise.