Dog barking and growling at visitors

[CushionFiller]

Hoping some one can suggest how I can train my dog not to bark and growl at visitors. She is a nervous dog, and it looks like nervous barking/growling rather than excitement or aggression.

She is a medium size so it can look a bit intimidating.

Softest dog ever with immediate family.

What can I do?

The behaviour can last 5 minutes or so. Today she put herself into her crate, but continued to growl quietly, so I'm sure it is nervousness that is the problem

Has she always had this problem?

She has always been very barky but the growling wasn't happening before.