Vienna is much quieter than most big European cities I know. The general noise levels are way lower than in Paris or Rome for example. This is especially the case at night. Generally there is no noise at all at night; streets are starting to empty as early as 8pm (20h), traffic being very reduced.
However, Vienna is also a city that wakes up early in the morning. At 7am there are way more people and cars in the streets than at 8pm... 7 am is also the usual time when workers start their day on construction sites. And of course they will start first thing by doing the noisiest thing possible, especially if you, dear visitor, are sleeping in the building next door :-)
Vienna didn't get that beautiful and clean by chance: there are dozens, hundreds of construction sites all over the noble districts (1 to 9) from April-May to September-October, every year, everywhere. Yes, this is also the season when one likes to sleep with the windows open. There is no escaping this.
What to do?
1/ go stay in an ugly place full of 50's buildings where no construction works occur. Ever.
2/ get some earplugs from a local pharmacy if there is a construction site near your lodging
3/ do what Viennese do: wake up early and get used to it
- Vienna is generally way quieter than most cities
- if there is noise, it is more likely to happen in the morning and in the summer