The house we recently purchased has a weird smell in the garage, what can we do to get rid of it?

It smells like cooked broccoli. Sometimes the smell is there and sometimes it isn't. The smell is more prominent when it has been raining outside. The smell does seep in to our house so I'm pretty annoyed by it. The owners were just finishing putting drywall up when we first looked at it. We thought it was the carpets which we had cleaned before we moved in, but getting down for a good sniff has let us know that they are not the problem. Since then we have sealed the gap in the garage between the concrete and the drywall and we have scoured the floor with a concrete acid wash. The smell is still there! I have no idea what this could be. Any suggestions on what it is or what I can do to rid my garage of this stink would be greatly appreciated! Thanks for your time.