Rugs, Carpets and Carpeting - Possible bad Carpet Seam ?

I just had new carpet installed in a 16X20 room. The installers did a great job on matching the pattern at the seam and I am happy with the actual seam result. My big problem is that I can see a visible 3" line, I'm assuming tape, at the seam. This actually shows up better in artificial light, at night, than with sunlight. The installer said it should go away after it gasses out, but that was a week ago. The carpet is a patterned loop and pile that has 1" or so square non cut loops and then cut loops for the rest, like a berber hybrid or something. Is there anything I can do about this 3" visible line in the middle of my room?
Thanks in advance.

I would give it a little while to gas out like the installer said. If its a discolored yarn (yellowed) then he probabbly over heated it, but this is almost impossible to prevent. The other problem could be that he over stretched it with the length of the seam if you see that it is slightly lifted, and he may need to stretch it more perpendicular to the seam. Also if it is a lower pile carpet again this might be unavoidable and is the nature of seaming cut and loop low pile carpets. As long as its put together with no more than a 1/6 of an inch gap between the backings and he seam sealed the seams and it's not falling apart I wouldn't stress out too much and chances are that overtime with temperature change and foot traffic this will work its way out. Good luck.

Overheating a seam is quite possible to prevent.

Most likely where the capet hot melt tape, does not allow the carpet to stretch in that area the hotmelt grips, keeping it full, instead of elongated and stretched.

Cut a rubberband in half. Take a small piece of scotch tape and lay it sticky side up on the table. Take the cut ends of the rubberband and push them down onto the tape so the ends you cut are touching. Now, pick up the assembly and pull on the loose ends of the rubberband ever so slightly and observe.

Overstretched? You must be kidding. Most carpet installers do not give a residential carpet enough stretch.
Off gassing? Again, must be kidding. Please enlighten me to what is off gassing from a carpet seam.

There are a couple things that could be going on here. The first is a peaked seam. This is probably not your issue because you say it is three inches wide. That just happens to be the width of your garden variety seam tape. The cushion you selected may take some time to compensate for the thickness of the seam tape.

It is also the same width of the seam iron. That means that this particular area has been heated and manipulated by hand and with tools. One of these tools was probably a star wheeled seam roller. These tend to distort the nap of the carpet. This may never dissipate on its own and may need to be serviced by someone who has skill with steaming out this problem. This requires a steamer, a seam weight and an acquired skill. I would not recommend trying this as a DIY repair. Call your carpet dealer and hace them take care of it.