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This ounds like inefficient combustion. It can be caused by: Clogged orifice(dissassemble the oil orifice and clean ) 2. Bad or weak ignition spark( replace transformer) 3. Clogged oil filter(replace filter).
I would clean the orifice first, then try the oil filter. The weak spark can cause the unit to not fire up right away causing the unit to misfire and rumble.
If the first two do not fix the issue I would then replace the ignition tranformer.