House batteries not charging off engine

Batteries charge from generator, solar and converter but when running the engine I here a cluncking sound coming from a black box under the hood where the isolators are located. It used to be I would run the engine in the morning before running the generator. I would turn on the light over the dash, watch it. I would here the clunk and the light would turn bright for a second then back to dim. This would occur a couple times until the last clunk and then full bright. I then would start generator. Now the clunk occurs over and over again but does not go full bright anymore. Is it the black box or one of the isolators ? Thanks in advance. I know someone will have the answer. I try to do things myself before taking to an expert so any help would be great.