rattling noise at 1500 rpm

I have a 2005 Nissan Altima 2.5 S. I very recently (1 month ago) had to replace the engine because of bent pistons and stuff. The older engine was 100k miles run and the replaced engine is 79K miles run. The problem is - when i drive and the needle in tachometer crosses 1500 rpm i hear a rattling noise. This noise does not occur before or after 1500 rpm. It occurs exactly on that mid line between 1000 and 2000 rpm. What could be the problem here...let me know if you need more details please.