Hey sayfri,
I'm sorry you're running into this keyboard issue. If the ` symbol were continuing to print even after another key was pressed, I would think that maybe the key is somehow getting physically stuck down when pressed. It seems that this may be an electronic issue though since pressing another key will stop it.
It's worth a look though. Visually inspect the key as best as you can to see if you find anything stuck underneath it. You may want to use a can of compressed air to blow any dust or other debris out from underneath it. You could also use a clean paint brush to try to remove anything stuck down there.
If after that it is still happening, you may need to replace the keyboard. Here is your Maintenance and Service Guide. Keyboard part numbers begin on page 52. The U.S. keyboard part number is 699497-001.
Let me know if you have any questions :)