Noise on 40 meters varies a lot depending on your local noise level and the current atmospheric conditions. If you live in an urban environment you may have a lot of man made noise. This is usually easily distinguishable from atmospheric noise. Storm conditions greatly affect the amount of noise you hear. You can hear storms in any location if there is propagation to that location, but the biggest effect is from storms that happen to be close to you. I live in a quiet rural area. Last night with the beam pointed at Europe the noise level was below S1 on 40M. Tonight, pointed at Europe there are a lot of crashes going up to S9. That is because the beam is pointed directly into a storm over New England. You will also notice, in general, there is more noise during the summer than in the winter. That is because there are more storms in the US during the summer than in the winter.
The next time you have a lot of noise, go to one of the weather sites and look at a radar map of the US. Also check out this link that shows lightning distribution throughout the world, during whatever time you select.http://thunder.nsstc.nasa.gov/data/OTDsummaries/