Case Feed updating Modified Date

Hi

I have an existing process which when an email is sent associated with a case it will update the Modified On value to the time the email was sent. This is useful as it allows us to track that a case is being updated.

I would like to include the ability for the Modified On date of the case to be updated if someone adds a Feed note. 

I have however not been able to find a way of adding a source signal, which has the filter in it to only be for feeds added to cases.

Anyone able to give me a steer on how I can achieve this please.

thanks