One of the most common smells, and perhaps one you might not expect to encounter, would be the subtle hint of sulfur dioxide. Lead manufacturing was a major industry for much of the medieval period; lead was in huge demand for windows, roof tiles, guttering, pipework, pewter metalwork and myriad other uses, and areas of lead mining and smelting, such as the Peak District, were heavily industrialised centres. A major byproduct of lead smelting is sulphur dioxide; today it's captured and distilled to produce sulphuric acid, but medieval lead smelters weren't always so discerning. A recent study between the University of Leicester and the Harvard University Geology Department found that there were identifiable layers of medieval English lead pollution in ice cores from glaciers in Switzerland, some 1,500km away. Interestingly, particularly thick deposits can be roughly correlated with events such as the arrival of Gothic architecture and the construction of specific cathedrals, when the demand for lead was at its highest. Medieval burials from the Peak District often show direct traces of lead pollution, and while your average English peasant is unlikely to have been similarly affected, the pollution layer was sufficiently heavy that you may have been able to smell it.

In your typical agrarian village setting, your other most common smell is likely to have been woodsmoke. Rural housing was typically single storey, perhaps with a mezzanine level for sleeping, and while there was ventilation for smoke to escape, it typically wasn't a proper chimney, so interiors would typically have been quite smoky.

Textual evidence suggests that what people might think of as the more stereotypical 'medieval' smells - cow dung for example - might not have been as common as you would think. Sheep were a major cash crop in Medieval England, but would have spent the vast majority of the year out to pasture, as would cattle, and the majority of the cattle herd would be slaughtered in November. Oxen were critically important animals as they made up the plough teams central to the agricultural economy, but evidence suggests that while these were owned individually, they were kept communally by specific oxherds, who were typically paid a fee and for food by other members of the community. What textual evidence does suggest might be common is the smell of fish. Fish and eels were extremely popular foodstuffs throughout the medieval period; they were cheap, readily available in markets or through fishermen, and could be eaten on fast days when meat could not. Especially on fast days, therefore, fish might have been a very common smell.