sounds like the window regulator to me, happened to mine about 6 months ago, you can get a cheap diy fix kit off ebay for about £10 but unless youre pretty handy it will be an hour or 2 work for a mechanic to fit as you have to get the door off and disconnect the air bag etc. depending on the garage they may also only want to fit BMW parts which means about a £250 job if you use stealer prices