Hi Otter8!
Yep, it can be a struggle to find privacy to enjoy masturbation without being concerned about someone hearing you or accidentally interrupting! And it doesn’t feel as good if you aren’t relaxed. People masturbate in a wide variety of places - it’s mostly about where you feel comfortable! (The exception is, it’s not a good idea to masturbate in the same room as others at the same time without their consent.) Bathrooms in your home can be a good place mostly because it’s more likely to ensure privacy - you could also try while in the shower. As far as making sure others don’t hear you, if you have access to a fan or a way to turn music or tv on, that could mask some sounds. It’s also more likely that you perceive yourself as much louder than other people would because you are more conscious of any noise you are making.
If you want to bring up masturbation with your mum, it would be helpful to identify what your goals for the conversation are. For example, do you want to talk to her about having more privacy or do you want to talk to her about questions related to masturbation? It sounds like a good place to start would be to ask her if there are ways you can have more privacy at home - a way to shut your door, a way to signal you don’t want to be disturbed, access to a fan (this can be unrelated to masturbation, too).
This article
“About that talk with your parents” gives a good framework about talking about puberty, sex, and sexuality with your parents if you want somewhere to start!
I’m glad you were able to talk to someone else about masturbation! To your question about whether or not it is “normal” to masturbate next to your friend, whether we consider something “normal” isn’t a great indicator of whether it’s something we would actually enjoy or not (ie drinking coffee every morning can be considered “normal” but I don’t enjoy caffeine so I don’t do it. Eating soup for breakfast might not be considered “normal” but I love it, so I do!). As with all decisions about our bodies, I would encourage you to make sure that masturbating with your friend is something you genuinely want to do and not just because he asked you to. Does that make sense? Do you want to talk through how you’re feeling about it?