I've soldered a couple of SMD capacitors to pads which were not intended to host them. As a result there's a bit of space between the PCB and the capacitors. I have a signal on the capacitors as shown in this question on the output of the circuit and if my sound-card is right, the capacitors are producing sound of same frequency as the signal.
My question is: Is that a reason for concern and if yes, how would I solve that problem?
Most ceramic capacitor dielectrics show piezo-electric effect, which causes them to vibrate with the applied voltage. If that voltage has an audio frequency the vibrations may be audible.
That's no reason for concern, hundreds of billions ceramic capacitors are produced each year which show this effect, and which perform fine on the PCB.
The sound can be reduced by using smaller packages. In larger packages stretching/shrinking the PCB may act as a soundboard. With smaller packages this effect is smaller.
Note that there are no measurable piezoelectric effects in Class 1 capacitors, such as C0G or NP0 - neither of which is considered ferroelectric.
Further reading
Piezoelectric effect in ceramic chip capacitors
