What is it?
This is not about noisy neighbours, nor specifically about noise of any sort. It is all to do with being allowed to use the property you have rented without the landlord interupting that use.
A tenancy agreement grants exclusive occupation rights to a tenant. Whilst the tenancy is in existence, the property is the tenants’ home, and as such they have the ultimate right to admit or deny access to anyone – as would any householder. A tenancy agreement may purport to give a landlord a right to enter the property – at will, or more usually with 24 hours notice, but this can not overrule the implied right to quiet enjoyment, which has been in common law for many centuries.
There are exceptions to this, the landlord can ask the court to order you to give him access - this is more likely to be successful if the contract has an access clause. Without a court order, the landlord can demand access to complete any works that the law obliges him to conduct - for instance, gas safety checks, roof repairs etc. However, as these are specified in law, they are to protect the tenant and allowing access should be to the tenants benefit.
In the real world, what does this mean?
Essentially, that no one, not the landlord or his agent/employees may gain access to your property without your express permission. If the landlord enters the property whilst you are absent (without your permission) that is a breach of the covenant of quiet enjoyment. If the landlord is banging on your door, making threats because you are late with your rent, that is a breach of the covenant of quiet enjoyment. If the landlord wants to sell the property and agents/purchasers view the property without your permission, that is a breach of the covenant of quiet enjoyment.
What action can I take
Really, prevention is better than cure. If you feel the landlord/agent is breaching your rights, you should write to them (keeping a free certificate of posting) explaining your concerns and your rights - together with your suggestion for a solution if at all possible.
You could also consider changing the locks, fitting a door chain or an alarm to prevent unauthorised access.
Finally, you may consider legal action for breach of contract, harassment or attempted illegal eviction. Your chances of success would depend very much on your individual circumstances, so you should obtain legal advice before contemplating such actions.