Because of the tone of the instrument or human instrument
harp
Out, because it is lengthening the instrument and therefore making it lower, or, flatter and making it no longer sharp.
micropipette
Jews harp
A knife.
knife I agree Tori 112
The word "knee" rhymes with "see" and has "kn" at the beginning.
knell
Knoll.
knife
Cutting instruments are often sharp because a sharp instrument is better at cutting than a dull instrument is.
A sharp instrument hazard is a symbol that means if you see it you should handle you equipment with care and watch out for sharp edges.
Would "knoll" satisfy?
One example of a warning sound starting with "kn" is a knell, which is the sound of a bell ringing solemnly, often to mark a death or funeral.
It's called a knapsack.
Some words beginning with KN:KnackKnackerKnackwurstKnapsackKnapweedKnarKnaveKnaveryKnavishKnavishlyKneadKneadedKneaderKneadersKneadingKneadsKneeKneecapKneecappedKneecappingKneecapsKneedKneeingKneelKneelingKneelsKneesKnellKnelledKnellingKnellsKneltKnessetKnewKnickerbockerKnickerbockersKnickersKnifeKnifedKnifesKnifingKnightKnightedKnighthoodKnighthoodsKnightingKnightlinessKnightlyKnightsKnishKnishesKnitKnitsKnittedKnitterKnittersKnittingKnitwearKnivesKnobKnobbierKnobbiestKnobblyKnobbyKnobsKnockKnockaboutKnockdownKnockedKnockerKnockingKnockoutKnocksKnockwurstKnollKnollsKnotKnotholeKnottedKnottierKnottiestKnottingKnottyKnowKnowableKnowingKnowinglyKnownKnowsKnoxKnoxvilleKnuckleKnuckledKnuckleheadKnucklingKnurlKnurledKnurling