It could either be the charger circuitry in the base unit, or the power unit.
I assume that the beeping stops if you unplug the power unit to the base charger unit?
you assume correctly
Then its the charging circuitry inside the unit which is causing the problem. You may find that it varies depending on whether its charging the phone batteries.
Normally any beeping would be quit a high frequency, that may only be audible to younger people, but a fault condition could cause this to be of a lower frequency.
If the phones are new, you could send them back.
I think we've had them about a year.
Thank you for your help anyway
You may be able to get another base charging unit, from an approved spares supplier. Try the helpdesk number in the manual, they may be able to give you a number.