We recently purchased our first home and were thrilled to find out that it had a tankless water heater as we have heard all the great things about them. However, something that I haven't read on any of the websites, is discussion of how noisy they are. Is this just our model by chance? It is a Bosch and the home is about 11 years old.
We have a 2 story house and when you turn on the water - you can hear a low rumble throughout the house. Not a major issue until you have a house full of guests and water is getting turned on and off constantly - it makes for an annoying situation. Any thoughts, ideas, comments are welcome. If it is our model, can we simply replace it?

A lot of noise issues with water systems are caused by incorrectly supported pipes. any noise that is transmitted from a fixture regardless of it being the tankless unit or a failing valve on a faucet will travel through the pipes which (no pun intended) sounds like what you have.
I would look for the install manual that came with the heater, or perhaps look it up on the internet and check and see if its properly installed. These units should be and remain fairly quite when running. You will get some burner noise when it operates but it should not be so loud that you hear it all over the place.