Learn Dutch: Het lawaai is enorm, zij slaan namelijk uitbundig op de trommels. - The noise is enormous; you see, they are hitting the drums exuberantly.

You could combine the ideas by saying: Al slaand op de trommels maken zij een enorm lawaai (While beating the drums, they make an enormous noise). The original sentence uses two separate clauses to explain the situation, but the al + present participle structure is a great way to emphasize that the drum-beating and the noise-making are happening simultaneously.

Why is namelijk translated as 'you see' here, and could we just use want?

Namelijk is an adverb used to give a reason or explanation for the statement that just preceded it. It translates well to 'you see' in this context. You absolutely could use the conjunction want (because/for) instead, but it would replace namelijk rather than sitting in the same spot: ..., want zij slaan uitbundig op de trommels.

Why do we say op de trommels slaan instead of just de trommels slaan?

In Dutch, when you play a percussion instrument or strike a surface to make noise, you use the preposition op. If you were to drop op and just say zij slaan de trommels, it would sound like they are physically attacking or beating up the drums as objects, rather than playing them.

Why does namelijk come exactly where it does in the second clause?