Electronic – Electrical noise sources in farms

There can be a lot of devices operating in the 433 MHz band. Many cheap radio devices (like wireless thermometers and stuff) operate on 433 MHz.

The house construction could have something to do with this as well. Did you try testing it outside (with a line-of-sight) near his house?

Noise is difficult to measure, and the amplitude you see on your your scope is only a first indication of the level.
Do you want to measure absolute noise levels, or just comparative? In the latter case the scope could be a good instrument, but at the given levels the average $500 scope will have so much noise itself that any measurement becomes in fact meaningless. You need a high quality scope + ditto probes to do this.

The difficulty with measuring noise is that it has a wide bandwidth continuous energy spectrum (the continuous spectrum makes it hard to separate noise from signal, notch filters may work). Ideally you measure the noise energy through RMS-to-DC conversion. This is not for the faint-of-heart, as your RMS-to-DC converter has to be very sensitive due to the low levels, and wideband. And of course be low-noise itself! Liquid nitrogen helps :-).
In any case, absolute signal-to-noise ratio is not as easy as reading amplitudes.