On the very first page of the "Reducing Background noise" tutorial, the second sentence starts with: "Though this effect[s] can...". Instead, it should probably be written as "Though this effect can" (in singular form).
Also, in that same tutorial, on the page whose heading is "Tips and tricks for even better results", there are two errors:
1) In the first asterisk, the last sentence partially reads: "...if noise reduciton is applied". The word "reduciton" should instead be written as "reduction".
2) In the third asterisk, the paragraph ends with two periods, whereas it should probably only end with one.