A grunt can make just about anything you do suddenly extremely manly. Upon entering the tub, let out a grunt from your lower register and anyone within earshot will know that a manbath is occurring.
When you grunt, make sure you find the proper balance of all the types of sounds within that single grunt. One grunt, or series of grunts if need be, can contain a complex array of tones, as if you're producing music. Play around with balancing how deep and gutteral it is with rough tones, smooth tones, length, strength, and more. Eventually, you'll be a grunting master and may be tempted to use it as your primary method of communication. Do so at your own discretion, and consult with those around you to ensure you aren't being too creepy.