why is having the money to reinvest in property an issue?
selling because of noisy neighbours is an extreme move given the transaction costs involved. How long have you had the property and how much of the mortgage is outstanding? If you have touble filling the property can you cover the mortgage out of your own funds?
Do you know if the nosiy neighbours rent and if so can you approach their landlord directly?
Have you made an official complaint to the guards or the council? If so you may be olbiged to disclose this fact to any potential buyers. Moreover, you tennants would probably be inclined to let any prospective buyer know about the noisy neighbours.
I empathise with your tennants as noise pollution of this type is horrible to live with. There must be a better solution than selling up. What type of noise is it? Does it go on all day?