It makes sense that major 3rds didn't bother him. With major tenths the fifth harmonic of the lower note is beating against the second harmonic of the upper not

It makes sense that major 3rds didn't bother him. With major tenths the fifth harmonic of the lower note is beating against the second harmonic of the upper note. With major thirds it is beating against the fourth harmonic of the upper note. The second harmonic is generally louder than the fourth, and so the effect is more noticeable.