Why Q signals are three-letter shorthand from international radio practice; QRN reports natural, atmospheric noise such as static crashes from lightning. It is the companion to QRM, which is interference from other stations or man-made sources. Sent as a question (QRN?) it asks whether the other station is bothered by static, and as a statement it reports that you are.
Watch out The choice about sending more slowly is QRS, and stopping transmission is QRT, both common Q signals that get mixed up with QRN on the exam.
QRN = Noise from Nature (static); QRM = Man-made interference; QRS = Slow down; QRT = Terminate.