Furnace

Have you checked you fridge propane exhaust tube/ stove top for any soot? Regulator on it way out?

It is a Atwood L.P. Gas Water Heater MPD- 93756

You have some sort of blockage in the burner tube or combustion chamber. Most likely, you have a wasp nest in the combustion chamber, second likely, a spider's nest in the burner tube. A good cleaning and inspection should cure it.

Actually along with a thorough cleaning that vintage water heater probably also has an adjustable mixer tube. Look on the burner tube for a small screw. Loosten it and with the burner running slide the outer tube a bit one way to see if the flame cleans up. You want it to sound like a jet engine. When you get it right you will know by the sound and the color of the flame.

Simply, the combustion system needs cleaned.

W/O BRAND/MODEL the following is 'generic' maintenance

Clean the burner tube...small bottle brush
Clean the gas orifice....soak in alcohol then blow dry (just blow in it..not air compressor)
Clean combustion/exhaust 'U' Tube.....run wire in thru combustion out exhaust/tie rag onto wire then pull it back thru (chimney sweep)

Reassemble
Fire off flame....adjust air shutter (sliding sleeve with slots) so flame is BLUE and slight roar
IF the MODEL has an air adjustment)

I am sorry, about my confusion...I meant to say water heater not the furnace....it is the water heater, which runs by gas

Furnace is running rich, it needs more air. Since you didn't tell us what furnace I don't know if it is adjustable. A good cleaning and possible adjustment is in order.

Black Soot is generally a sign of incomplete combustion!
There are trouble shooting and service manuals available on line !

I had to replace the burner screen on our older southwind. It had simply burned a hole in itself!

Whenever it turned on there was a loud whoosh,almost an explosion sound!

A simple repair ,a thorough cleaning of the burner tube and accessible parts got it back to 100% !