There's a phrase in German language, "eine harte Nuss zu knacken haben", literally "to have a hard nut to crack".

Maybe that means that he'll be cracking nuts (problems) even after he's buried?

I think it's more aimed towards his tendency to go against the (left) mainstream of his day and start fights - he argued for a delayed or stopped reunification of West and East Germany, he often criticized people for not dealing with their Nazi past thoroughly enough (which bit him when he disclosed his own previously secret Nazi past just a few years ago), he always worked in politics (mostly supporting the SPD).

"Wenn es dann kracht,"

you can also translate as

"where there's a fight"

so there's a bit of a double-meaning here - "krachen" means that there's noise from his crushing nuts, but it can also mean that there's a conflict [1]

[1] http://www.duden.de/rechtschreibung/krachen "(umgangssprachlich für streiten)"

I tried to convey this with gnashing, which is also something that literally happens when you chew nuts as well as idiomatically when you fight, when you gnash your teeth out of anger.

I would agree with that idea. The bag of nuts may represent the issues he has concerned himself with over the years or they may be future issues in need of a solution. Grass has always been very outspoken, very political and, more often than not, provocative. I believe he is writing about his legacy. He is convinced that his works will outlast him and that he will continue to have an impact through them even after this death. One could even go a step further and argue that he is saying his works are him (or at least his essence) and therefore he will continue to live on. The sounds coming from the grave may either represent the cracking of nuts, meaning solving the issue or they may represent the sound of fighting over said issues stimulated by Grass' ideas.

I didn't even think of that, that's a very different take on the poem, and possibly a more appropriate one, thanks!