Ok lets make the knot. Select the Radial Marquee tool and make an oval shape
on the image somewhere. Then Feather the image: Select>Feather. Enter about
10 for the amount.
Next, Filter>Distort>Wave. Try your settings with a default one.
When we applied the wave filter, our knot becomes slightly blurred and doesn't
fit in with the rest of the wood. To fix this, while the knot is still selected ,
Go to Filter>Sharpen>Sharpen Edges. Do again if needed. Now the sharpness is back,
this makes it look real.
Open a new file. I used 150 by 150 pixels. Now fill the background layer in with brown.
Add noise: Filter>Noise>Add Noise. Amount 60, Gaussian doted and Mono checked.
Motion blur: Filter>Blur>Motion Blur. Angle 0, distance 614.
Add noise once more, this time enter 8.
Ok lets make the knot. Select the Radial Marquee tool and make an oval shape
on the image somewhere. Then Feather the image: Select>Feather. Enter about
10 for the amount. 
When we applied the wave filter, our knot becomes slightly blurred and doesn't
fit in with the rest of the wood. To fix this, while the knot is still selected ,
Go to Filter>Sharpen>Sharpen Edges. Do again if needed. Now the sharpness is back,
this makes it look real.
Deselect.Ctrl-D. 