Add capacitors between each power rail and ground. They'll act like short circuits to the high-frequency noise. I see that you have one between the -9 V rail and ground-- that's a good start.
The type of capacitor you use can make a difference. Electrolytic caps, for example, are relatively cheap, but they only deliver their rated capacitance at low frequencies, so they're not as good for filtering out high-frequency noise. I've had good luck with tantalum capacitors in this kind of application, but be careful that you don't plug them in backwards, as they will explode. Be double careful with the negative rail, because "backwards" might be the reverse of what you think. For both rails, you want the plus on the capacitor connected to the more positive terminal, which is ground for the -9 V rail.
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 terms "power supply pumping" is the return of energy from a load to a power supply via output switches so that the power supply is forced to accept energy via an unintended path. This may lead to eg voltage rises that cannot be controlled by voltage regulators as the energy input occurs after the regulator stage.
In the context that you are asking, if you read the paragraph headed "Supply pumping" in the datasheet referred to in your accepted answer to your prior question -
Multiple audio inputs driven by one MP3 player output - it tells you exactly what you want to know. It says
One issue encountered in single-ended (SE) class-D amplifier designs is supply pumping.
This phenomenon is most evident at low audio frequencies and when both channels are operating at the same frequency and phase. At low levels, power-supply pumping results in distortion in the audio output due to fluctuations in supply voltage. At higher levels, pumping can cause the overvoltage protection to operate, which temporarily shuts down the audio output.
Several things can be done to relieve power-supply pumping. The lowest impact is to operate the two inputs out of phase 180° and reverse the speaker connections. Because most audio is highly correlated, this causes the supply pumping to be out of phase and not as severe. If this is not enough, the amount of bulk capacitance on the supply must be increased. Also, improvement is realized by hooking other supplies to this node, thereby, sinking some of the excess current. Power-supply pumping should be tested by operating the amplifier at low frequencies and high output levels