Another word for unnoticeable

engaged in conversation with some visitor to the engineroom, talks apparently undisturbed by all the multifold noise and rattle of the machinery, but let the noise alter in some item which, though unnoticeable to the visitor, betokens importance to the trained ear, and his passive attention is in a moment caught.

In this, the secondary spectrum is so much lessened that for all practical purposes it is unnoticeable.

Give the old needle and thread a try if you're handy; sometimes the result is nearly unnoticeable.

If this is the case, test an unnoticeable area to make sure that there are no chemical reactions before cleaning the entire ceiling.

Stick with neutral colors that are almost unnoticeable.