Troubleshooting can be very difficult sometimes to the person that has to physically do it.
You're there. You hear it or see it and sometimes the hardest thing to do is to tell it to someone else.
Another hard thing to do is to detach yourself from what you (think or believe so strongly in) to strictly what you can prove.
It all boils down to your specific belief system.
You filled it full of oil last week so it couldn't have been too low, so you didn't check that.

I.E., previously unknown to you, your friend on the back burnt their leg or foot on the exhaust on a long trip.
Then they went berserk and not realizing that they kicked the oil line damaging or kinking it on a long trip.

When you believe that anything could have happened to it, you stand a better chance of diagnosing the problem.
Below are some easy fixes and some very difficult ones as well as cheap and expensive ones.
Keep in mind an unhealthy bike doesn't care if you can't afford it or know anything about it. ¹⁾

All engine noises aren't good noises but it is a Harley and it's gonna be loud and it's gonna shake.
The front forks may appear to want to leave each other and your hands may feel a little numb after riding.
It's OK, it's suppose to do that.

See also Hunting Down Sportster Noises for help in diagnosing noises.

Engine oil is like blood in the veins. Running a bike too low or without will ultimately lead to engine seizure.

• High Oil Consumption
  • Worn/damaged piston rings ²⁾
  • Worn valve guides ³⁾
  • Improper chain oiler adjustment (if equipped) ⁴⁾
• Excessive oil leaks
  • Clogged up air cleaner breather hose ⁵⁾
  • Damaged gasket mating surfaces ⁶⁾
  • Gaskets loose from non-torque/ loss of torque ⁷⁾
• Excessive oil at crankcase breather
  • Damaged/ restricted oil lines/ fittings ⁸⁾
  • Leaking gearcase cover gasket ⁹⁾
  • Oil isn't returning to the oil tank ¹⁰⁾
• OIL PUKING OUT ENGINE BREATHER
  • WET SUMPING,
    This is simply oil from the tank draining slowly back down into the crankcase while the bike is parked for any length of time.
    When you start the engine, the excess oil in the crankcase is fired out the breather, onto the floor, (or into your air-filter on post-79 models).
    The oil should stop puking after the engine runs a few minutes and pumps the excess oil back up into the tank.
  • OVER-FILLED OIL TANK,
    If the puking starts after you top your oil tank, this is probably the problem.
    If you fill the oil tank to the Full mark while some oil has wet-sumped down into the engine, you have too much oil in the system.
    The oil from the sump will be pumped back up to the tank, dribble down the vent tube to the timing cover, from where it is fired out the engine breather.
    This puking will continue after initial start up until all the excess oil has been fired out, which can take a while.
    The cure is to drain a quart or so out of the oil tank, run the engine for five minutes til the puking stops, then top up the oil tank to the full mark.
    DO NOT be tempted to drain oil out of the sump by taking out the threaded drain plug under the front of the engine (if equipped).
    These are notorious for stripping the threads and are very difficult to repair properly.
    In most cases the plug is factory installed and is not intended to ever be removed.
  • WORN ENGINE,
    If your engine breather continues to puke oil or blow smoke after the above two things have been eliminated,
    Your problem is most likely wear in the cylinders and heads.
    Worn rings and even valve guides, can allow blowby of combustion gasses into the crankcase area, which then comes out the breather.
    Usually this will be accompanied by smoke or oil coming out the exhaust pipes too.
    A compression test will give some indication of top-end condition.
    Anything below 120psi is suspect, according to the factory manual.
    These bikes will still run ok at even 100psi, but they will be down on power and consume oil, and blow fog out the breather pipe. ¹¹⁾
• Oil isn't returning to the oil tank
  • Damaged/ restricted oil lines/ fittings ¹²⁾
  • Oil pump not working properly ¹³⁾
  • Engine too low of oil
  • Oil pump Scavenger gear woodruff key sheared off/ broken ¹⁴⁾
  • Stopped up oil filter
  • Faulty check ball at filter housing
  • Faulty oil pressure regulator
• Engine Overheating from excessive load
  • Engine oil level too high.
    Adding too much oil to the oil tank/ engine causes undue pressure in the crankcase which degrades engine performance and increases heat ¹⁵⁾
  • Oil viscosity too high.
    Using heavier than recommended oil can damage the oil pump, oiling system and cause engine to drag and overheat ¹⁶⁾
• Engine Overheating from poor lubrication
  • Engine oil level too low which increases friction
  • Wrong oil viscosity/ degraded oil viscosity
• Change in Engine Vibration due to degraded oil
  • Vibration analysis can pick up subtle vibration changes.
    You also may be able to identify lubricant starvation before the extreme heat from friction is discovered. ¹⁷⁾
• Engine running poorly on acceleration
  • Oil viscosity too high.
    Using heavier than recommended oil can cause engine drag
• Engine running poorly with continuous high revs
  • Wet sumping occurs when the crankcase fills up with oil.
    The crank has to turn in an oil bath when it's really designed to run in open air.
    The crank running thru the oil heats it up and adds lots of bubbles.
    It also really cuts down on the available power.
    The scavenge pump picks up the oil and sends it to the oil tank.
    The hotter the oil, and the more air bubbles in the oil, the harder it is for the scavenge pump to move the oil.
    So, the oil collects in the crankcase even more. ¹⁸⁾

The tables below have 2 separate but equally important columns; Possible causes and Possible affects. It is just as is important to know what happened as it is to know why it happened. I.E., a broken piston ring can be fixed but if the cause was low oil pressure and that was not addressed, it'll happen again or worse.

Transmission noises are usually not what you want to hear but there is a natural symphony that happens sometimes when changing gears. The harder the push is on gear shifter, the harder the shift forks smack the gears around and the noise is more audible than if shifting easier with a quick smooth motion which sometimes sounds like a light hollow clinking as in two plastic parts shaken in a sandwich bag.

A Sportster is gonna rumble and vibrate due to the engine configuration. New or distinct vibration can be a sign of problems however.