Some noise may be heard especially with single coils, which amplified by the booster is more audible. Single coils are notorious for being a bit more noisy than humbuckers which have signal phase canceling from the second coil. If you want to check the noise of the pickup, just put the booster in bypass and raise the amp input gain to the point of booster's amplification, and compare. However, you will hear less noise from the amp because it is better shielded anyway, but you will understand what the booster amplifies in place.
Here's what can help containing the noise:
- A proper shielding, not one made with "conductive paint". If you ask a proper trained technician with electronic engineering background, you will find that an effective shielding of the electronic circuits can be done only with iron cage, of a certain thickness. The next practical option for guitar is cooper foil (if it is sticky type, be sure the sticky tape is also conductive!). Shield also pickup's cavity and be sure to shield the plastic pickguard as well. Paint shielding is "effective" up to a point and is "most effective" with humbuckers, which are already quiet due to hum canceling coils. As a last option, use aluminium foil.
- Connect all ground wires in a single point which makes contact with the shielding.
- Reduce the length of every wire in the electronic cavity before the booster, because every wire is an antena. Anyway, try to push them away from the small electronic parts on the circuit board of the booster.
- To investigate aditional noise sources that are amplified, temporarely stay away from the guitar amplifier and other electromagnetic sources such fluorescent lights or tv or power lines - they all radiate noise which is picked up by the pickups and the wires in the guitar, which amplified, will start to be heard. Also change the position of the guitar in relation to these sources, to hear the influence on noise; there might be positions with less noise, so you will understande the source and how the pickups and the wires inside guitar picks it up. Some more noise will be heard if you don't touch the strings with guitar volume open or in a certain position. Amplified, this noise is more obvious.
- Play with booster's gain from the minimal level; raise it up in small increments and test the effect of it, when compared to bypass levels. Use only the amount of amplification needed to push the amp or effects in front of the amp, if you have any. Most likely, after a certain point, you won't hear much difference in amp's response, especially on higher gain channels, because of their natural compression. Speaking of pedals after booster and in front of the amp, especially if you have distortion pedals, they will also add their noise and they will amplify even more of what is in front of them. Bypass them temporarely to check the added noise.
That is pretty much what can be done to contain an existing noise which is there from the start anyway, but then, amplified, will become more obvious.
At playing levels, say in a band, these noises are usually covered and could be ignored during rehearsal or live performance. In between... there is the noise gate pedal.
Also, keep in mind that not all single coils behave the same, due to their construction materials and frequency response. With different pickups you will hear different noise/things. Try the guitar and the booster with a different amplifier as well, just to compare and draw a conclusion.