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is normal that a bass guitar that is not connected has a strong buzzing of the strings?
To check if it's the truss rod, press the E string down at the first fret with your left hand and hold it. Then, press the E string down at the last fret with your right hand. This should open up a gap between the strings and the 7th and 12th frets (about the thickness of a credit card). If the curvature of your bass neck is not correct, you need to adjust the truss rod.
To adjust the action, turn the screw in the truss rod to change the curvature of the neck. If the gap is too large, insert the Allen wrench or Phillips screwdriver into the screw (located either on the headstock or at the other end of the neck) and then tighten the truss rod by turning the screw clockwise. If your strings buzz when you play on the first four frets (near the headstock), you need to loosen the truss rod by turning the wrench or screwdriver counterclockwise.
Your bridge may also be too low. Measure the string height at the 12th fret from the top of the fret to the bottom of the string. For a middle-of-the road action, set the bass side of a 4-string (the E string in standard tuning) to 7/64″ and then set the treble side (G string in standard tuning) to 5/64.
Lastly, if you are dropping you are down-tuned to something like Drop-C, but with lighter gauge strings, you'll want to get some that are a bit beefier. (They'll also maintain their intonation better.)
I hope this information helps!