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D. Excessive noise.
This can often be a concern even if there is not a reduction in performance. The most common causes of noise are:
1.
Cavitation:
The jet unit is starved for water, and usually sounds like a rattle or a can of loose bolts in the back of the boat.
Most likely a blocked intake grate.
2.
Moan or whine:
The jet unit can exhibit some "turbine whine" not unlike a turbocharger noise but you will know what is usual
with your boat. However, if you have a new more obvious moan/whine, especially if it is a very low frequency grumble at idle,
that increases with engine RPM's then it is likely to be a rough/worn/water damaged thrust bearing. If water has gotten into the
bearing, it is usually as a result of a flooded bilge at some time on a warm bearing, then water can be sucked in as it cools.
3.
Periodic vibration:
Often at specific RPM's and disappearing at other throttle openings is probably a torsional vibration ema-
nating most likely from the universal joints on the drive shaft.
