HONEYDO: Stop squeak at the rear of the tread on the main stairwell. We'll use a countersink bit and bore a pilot hole through the back of the riser into the tread. Then we will drive in a 5cm screw to join the two parts and eliminate any friction. We will probably install four screws on two steps from underneath.
If that doesn't work, we'll go high tech and buy a Squeeeeek-No-More Kit from O'Berry Enterprises.
O'Berry Enterprises have offer a free video with their Squeeeeek-No-More Kit.