A sash that rattles is usually the fastener
I lay awake in November and the bedroom sash rattled. A hard, thin knock when the wind hit the back of the house. I thought the cords had gone. In the morning I shut the fastener. It turned, but it did not pull. The keep on the other sash had walked. There was a gap you could see daylight through. The sashes were fine. They were not held together.
A sash fastener is a catch. One part on the top of the bottom sash, a keep on the bottom of the top sash. When it seats, it draws the two meeting rails together so the window cannot rattle and the draught cannot get through the gap. If the keep is loose, painted shut, or no longer in line, the fastener spins and does nothing. People buy a new window. I move the keep.
A letterbox that howls is usually the brush is the slot in the front door. A loft hatch that dumps heat is usually the catch is the lid in the ceiling. This is the same family on a sliding pair. A catch that does not pull leaves a slot.
When I check I close both sashes and I turn the fastener. I want a firm draw, not a flop. I look at the keep. Screws in the meeting rail should bite timber. Screws in a crumb of old filler will walk. I look at the line. If the hook only kisses the keep, I mark a new place and I move it, or I pack it so the hook engages. I do not crank a fastener that has no keep to pull against. I will bend the handle and still have a rattle.
Paint is the other thief. A thick coat on the keep fills the hook. The fastener looks shut. It is sitting on paint. I pick the paint out of the keep, or I take the furniture off before I paint next time. A painted-in catch is a rattle you applied.
Skip a new sash because one keep walked. Skip a wedge in the frame as the only lock. A wedge falls out at 3am. Skip screws so long they come out in the glass rebate. Skip a fancy lock on a meeting rail that is rotten. If the rail is soft, I stop. The fastener is not a repair for dead timber. Skip forcing a sash that is jammed in the staff bead and calling it the fastener. If it will not meet, the fastener cannot draw it.
I also look at the other sashes in the room. One loose keep is a habit. I try them all on a windy night, not only the one that woke me.
UK year is autumn into winter, the first gale after you have had the windows open for months. I now shut the fastener and I push the meeting rail. If it still knocks, the keep has not got it. I do that before I buy draught strip by the metre. Strip on a pair that will not pull together is a strip in a gap that still moves.
I do not need a new window because the sash rattled. I need a keep in line, a fastener that draws, and timber the screws can bite.
If I later add a buy link here, it will be a paid promo.