Leaves are not right for common milkweed and arrangement of leaves isn't right either (opposite vs. alternate).

It reminds me of Crotalaria Here is a link that might be useful: Crotalaria

Are we talking about Laburnum? I grew up with them as they were a very common suburban front garden and street tree of the 20s - 60s. Yes, they are a bit boring out of bloom and all parts are toxic. But if well grown and left alone ie not brutally pruned or 'managed' they can be beautiful shapely trees. The wood it very attractive and sought after by carvers and turners. If you are going to get one, get a named cultivar, not a seedling. The latter can be scrawny and disappointing. Laburnum x watereri - 'Vossii' is the one with the very long chains.

First thing that came to my mind was bindweed (Convolvulus arvensis)which spreads by seeds & deep rootlike rhizomes. Can't say for certain without more info but maybe you could look up this weed. As I'm currently working on a final exam about weeds, I probably have resources to identify it - too bad you're unable to post a pic. Summer