I only just realised, after reading about pronunciation in Austro-Bavarian dialects, that I usually (i.e. when not specifically enunciating distinctly) pronounce a sequence of underlying nasal + stop + -en (shwa + alveolar(?) nasal) as nasal + glottal stop + same nasal—"Lampen" is [lam?m], "Enten" is [En?n], and "sinken" is [ziŋ?ŋ].
I never thought about the fact that there's a glottal stop in there (though I think I did realise that the -n in -en assimilates to the previous nasal in such cases and can surface as [m] or as [ŋ]).
On the other hand, if there is no intervening stop, then the sequence of nasal + -en turns into long nasal for me: "schwimmen" is [SvIm:], "rennen" is [rEn:], and "singen" is [ziŋ:].