My daughter is 3 but often (and we are sure, concisely) acts like a baby, making gurgling whining noises, crawling about etc. Apart from being very irritating sounds from a three year old we don't seem to be able to convince her to stop. Do you think this is a common phase that she will grow out of or should we be trying something else to stop her.
It's completely normal. She's just being imaginative. My daughter is 4, almost 5, and she crawls around acting like a puppy. Panting, barking, the works. It drives me nuts, but she knows that she is not really a puppy. I'm sure your daughter knows that she's not a baby, but she might think they are cute so she acts like them.
You could always praise her when she uses her big girl voice to talk to you by saying, "I like it when you use your big girl voice. I can understand you a lot better than when you are pretending to be a baby."
I'll give out high fives or thumbs ups when I tell my kids they are doing a good job with manners, using big kid voices, or remembering to wash their hands after going potty. Kids love attention!
It's possible that your daughter is craving the type of undivided attention that is often given to a baby (being held, nursed, cuddled). This might be especially true if there is a new baby in your life that you have spent some time with, but could still be true even if there isn't.
I would suggest indulging her when she does this. If she starts acting like a baby, pick her up, hold her like a baby and rock her or coo at her. Tell her how much you love "your little baby." If she actively resists, then my theory is probably wrong, but if she seems to enjoy it, then that's probably what she needs.
Either way, I wouldn't be concerned about it. Although it's annoying to you, there's no harm in it and she will eventually learn to act her age. But I suspect that actively discouraging the behaviour will probably make it last longer, especially if she is a strong-willed child, so indulging or ignoring it are probably better options.
Another option is to re-direct the focus during her "big-girl" behaviors. Speaking to her like a little person saying something like, "WOW!! Give me a five! That was awesome! Man, your turning into a big girl, I'm impressed!" and while being overly emphatic about it... will teach her that new responsibilities are awaiting, the both of you. AND, you're in it together; which will also build her self esteem and confidence. Aside from the messes initially caused by teaching new skills, it could end up benefiting you with a little helper. Additionally, you could introduce a baby doll to the situation when she is acting like a baby and try to shift her thought as to what her baby thinks... ie. go grab her baby doll and make a loud whiny baby noise - saying MAMA, MAMA and replicate how she is acting towards you and see how she responds; then go from there as I believe she is trying to communicate with you, but doesn't know how. All girls I know, love attention from their mother and father. :D This too shall pass and you might even miss it one day. Good Luck!