bhm_to
Model #: Generac 20kW 0055251 Serial #: 5465087
Propane or Natural Gas: Propane
Length of gas pipe feeding the generator: 30 feet.
Size of gas pipe feeding the generator: 3/4"
How long since the last full maintenance:1 year
How long have you experienced this problem: 3 weeks 
Description of problem: When starting it makes funny noise like bicycle brakes on a rusty rim, at the end of each failed starting cycle. The generator does start. My guess is the starter is wearing out, but looking for comment from the experts.
thanks

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78buckshot
The starter is not one of the items we have to replace often, not saying that's not your issue but not real common. How many hours are on the unit?
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BrianMartin
Could it be a squeaking noise from the rubber mounts when the generator is rocking while it cranks? I hear this often enough after a generator is turned off, and the rocking back and forth as the engine slows to a stop causes some squeaking noises.. 

Video?
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Birken Vogt
The crankcase breather makes a reed-like noise when the engine grinds to a halt on these.  It is air pulses moving past a reed of some kind.  It certainly sounds like some bicycle noises I have heard.
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bhm_to
This unit was installed around 2009 I believe. 311 hours but I did put a new control board in years ago, would this have reset the statistic? The noise is before it ever runs and is on a failed start up. Like something is spinning internally? On exercise this Friday I will make sure I am near it. 
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78buckshot
With the added description I would suspect the starter, sounds like the solenoid is not pulling pinion gear fully into mesh with the flywheel ring gear. How old is the battery?, when it does start correctly does it sound like the starter motor is spinning the engine at a normal speed?
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BrianMartin
Or possibly the stepper motor whine?
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bhm_to
Heard it start on exercise last Friday. Started on second attempt so not a lot of time to get hood open. It sounded like it was on the left side of the unit, when facing it. Brand new battery, which is why I checked and updated the battery a month ago. Yes I think the starter is spinning the engine at the right speed? Not being a mechanic I also was thinking of fly wheel spinning or something like that. It is starting which is always my main concern. Haven't had an outage for around 3 weeks now, but you never know when the next one will be.  Stepper motor swapped 2 years ago, this was to fix a high speed running problem, been good since. 
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