In older scores you invariably see different customs of notation that have been outdated through the centuries. One misconception I often encounter is the timpani trill line. Seemingly quite innocent and actually not that ambiguous the trill line for timpani is however obsolete.

The trill is quite normal and seen quite often in particular in strings and winds. In any given key the trill means to play the written note and in an unmeasured manor interchange it with the diatonic note directly above it. Meaning the clarinets in this example would trill between E-flat and F for the first two beats and between D and E-flat for the next half of the bar.

The alteration to the trill sign mean that the diatonic note above the D is flattened.

This convention quite logically would mean that a trill line in the timpani would either have the player play on two adjacent drums tuned a second apart or have the pedal go bonkers!

Instead the symbol you are really looking for is the unmeasured stem slashes that indicate unmeasured repetitions (here the repetitions are written out in 16th-notes for clarity)

This is the ‘new’ practice of notation. Not to say that the other doesn’t exist as it has done for many years and can also be found in notating tambourine rhythms for example. In that case this:

Is still better than this:

since the tambourine is not alternating between two pitches.