I'd bet that if you disconnect the wire at the MOSFET then you'll still have the 1.3 MHz at the pullup resistor. If that is the case then your wire is acting like an antenna and you're receiving something.
Sources of radio waves tend to change. This could be why you haven't seen this before. Or, you never used this kind of wiring harness before and so never tried an "antenna tuned to 1.3 MHz" before.
There is not much you can do with this to fix it. At least, not much that doesn't have some negative side effect.
One solution is to put an RC filter just before the pullup at the input. Size it so the cutoff frequency is as low as possible without effecting the speed at which you can measure the cable. It might be that you can't do it without effecting the speed, those are the breaks.
Keep in mind that the R from the RC, and your pullup resistor will form a voltage divider. When the MOSFET is on, and the wire is 'low', then the R+Pullup will limit how low the voltage on the input will go. Make sure that the R in the RC is low enough, or raise the value of the pullup, so your input will go low enough.
Another possibility is that you are exciting some sort of resonance in the wire. This is unlikely since the speed of the signal traveling down the wire and back is much faster than 1.3 MHz. But if it were a problem then I would look at putting a resistor in series with the MOSFET. Basically something to slow down the falling edge of the signal.
One thing you could do is up your voltage, C batteries only give around 1.5, same as AA, so if you put multiple AAs in a series you can max your voltage as their resistance(0.12) is a bit lower than Cs
Best Answer
It's about shield capacitance to earth versus shield capacitance to the noise source aka electric field interference. But....
... If the near field probe were a magnetic loop, things would be different; it wouldn't make much difference where the aluminium sheet was placed because the mag field would get thru the same but, attenuated because of eddy current losses in the aluminium sheet.
So, back to electric fields AND importantly for this question, the aluminium sheet appears to be unearthed. When I say unearthed I mean galvanically unearthed. However, the aluminium sheet will still have capacitance to ground and that capacitance to ground will tend to make it act like a faraday shield. In other words it will block electric fileds and reduce noise pick-up to the near field probe.
If the aluminium sheet were close to the near field probe, then the electric field impinging on it is a bit weaker than if it were close to the noise radiating wire. This is primarily due to distance. This is important to remember.
This means you can regard it as having more capacitance to ground than capacitance to the noise source. Try this for an equivalent circuit: -
simulate this circuit – Schematic created using CircuitLab
In this scenario the noisy electric field trying to hit the probe is dramatically attenuated due to capacitance to earth dominating the capacitance between the noise source and shield. It's a potential divider made from capacitors.
Now, consider the scenario when the shield is really up close to the radiating noise wire. In effect, the shield becomes massively capacitively coupled to the noise source. It "effectively" becomes part of the noise source because the capacitance between it and the noise source is far greater than the capacitance to earth. There is some slight attenuation because there is still some capacitance to ground but basically the aluminium shield re-radiates the noise minus a dB or two.