It's really hard to tell from your description. A good description of the noise and when it happens would be a good start.
You already said that the noise happens when you play a note. What happens if the volume is turned down? Either using the control on the electronic piano or by using the pot in your circuit. If the noise goes away then you can assume that the problem is either with the signal level on the input, or that you're asking the amp to put out more than the rated 3.5 watts.
If I were a betting man, I would guess this is the case. You're using a 3.5 watt amp to power a 15 watt speaker. Normally for a 15 watt speaker you would be using a 15 to 30 watt amp. So your amp is undersized.
Adding a cap like what @Manmanguruman suggests isn't a bad idea, but I would hope that there is already some caps inside the amp module. Still, you would use the largest cap that is somewhat practical. Start with about 470 uF and higher.
The pot should have a log taper, not a linear taper. Changing the pot will make the volume control more useful-- but will do nothing for your noise problem.
It is also possible that you have a signal loading problem with the piano. Basically, the piano might not be able to handle having an amp plugged into it. I would hope the piano was designed better, but I have seen some products do some stupid things. Turning the volume down with your linear/log pot should help this issue.
Other than that, the only thing I can think of would be a component failure. Something like the speaker, amp, battery, or piano actually being broken in some way.
The problem is common to this type of audio system. I would bet if you looked at the noise spectrum you would see 60 Hz plus many of the harmonic frequencies (120 Hz, 180 Hz, 240 Hz, etc.). The fact that it is more than just 60 Hz, or 50 Hz in some countries, is an indicator that it is not just simple ground loops.
I would also bet that your laptop power supply has only a 2-prong AC plug-- lacking the third ground plug.
In this type of power supply, the output is electrically isolated from the AC input. But it is not perfectly isolated. There is a small amount of current that flows between the isolation barrier. This is called the "leakage current". It is not a lot of current, but it doesn't have to be.
Some laptop users report getting shocked or having a tingling sensation in the legs when using the laptop while wearing shorts! The reason for this is that leakage current is going through the screws in the bottom of the laptop and into their legs. It sounds dangerous, but the amount of current is well below the safety limit. It is more startling than anything else. If you are wearing pants then you're insulated.
Laptop chargers that have the 3rd prong on the AC plug do not have this problem because that third plug is connecting the laptop chassis shield to ground-- forcing that leakage current to go to ground instead of into your leg. Of course, there is no leakage if you are running off of batteries.
In your case, the leakage current is not just going into your leg, but into your radio receiver. The solution to this is to properly ground your laptop.
You will have to experiment with this a little bit to find the best solution. Getting a power supply with a 3-prong AC plug is the best, but not always possible. The next option is to find something on your laptop that you can ground. Make an adapter from that 3rd prong to "something". That something could be the signal-ground on the output cable of your power supply. It could be a screw on the laptop. Or a shield on an unused laptop connector. Or the ground/shield on your audio cable.
Make that 3rd prong adapter, but leave the other end bare for the moment. Then start poking it around to see if or where you can connect it and have the noise go away. Once you have found a place or two, then finish up the adapter so it is easy to use.
Two warnings when doing this: Make sure that whatever you are grounding is actually ground! On the power supply output, make sure you ground the negative or gnd conductor. And when poking around, understand that you might actually have to poke a little hard. Both the bare wire and whatever you are poking will likely have a thin layer of non-conductive stuff on it, and you need to apply enough force to poke through it. Rubbing sometimes helps too. The non-conductive layer is sometimes paint on screws, or an oxide (rust) on the metals.
Oops, here is a 3rd warning: Be super careful when making that 3rd prong adapter. You're messing with potentially lethal voltages and we don't want you to die. Build the adapter in a way that there is no possibility of it failing and shorting out against either one of the other two conductors in the AC plug.
Give it a try and report back what you found!
Best Answer
In general an SMPS generates a lot of switching noise. However, most SMPSes use a switching frequency above 50 kHz as a lower frequency would be inefficient.
An SMPS can generate lower frequency components depending on its design and in particular the amount of current it has to supply. Many SMPSes switch continuously at the same frequency. However some SMPSes go into a pulsed mode (PFM: pulsed frequency modulation) when the load draws only a low current. This is done to improve efficiency at low loads. The pulsed frequency can be in the kHz range, so every 100 us - 1ms there will be a short burst of 100 kHz. This you might want to avoid in your application. If the PFM mode of your SMPS is a problem then you might be able to work around it by drawing more power from it.
Another way to suppress the switching noise is to use linear regulators whith high supression in the desired frequency range.