Odd behavior '95 lumina

When it's cold out, the motor sounds like a diesel truck motor - it's VERY loud. It runs fine, pretty low milage (68k). What could be the problem? Any ideas? Is it something I should be worried about?

Without actually hearing the nature of the noise, not much chance for any meaningful help. Knocks, ticks, exhaust? How long does it last? Very vague query.

I just went thru this with my 95 3.1 L. At first I thought it was the belt tensioner pulley (one bolt and it's out), but that was not the problem. Once I had the belt loose, I spun the water pump pulley and it made that rattling sound. I changed it out and problem solved. BUT (I have a post here too) now the rad-fans won't kick in and I suspect an air pocket trapped in the upper cooling system. I've already gotten some good advice on purging the air with bleeder valves. I'll reply here if that solves my problem...wife has the car for the day.

Shot in the dark...real loud exhaust rumble that settles down to a ticking or quiet rumble as it warms up usually a cracked exhaust manifold or flange, or loose/broken manifold mounting hardware...

When it's cold out, the motor sounds like a diesel truck motor - it's VERY loud. It runs fine, pretty low milage (68k). What could be the problem? Any ideas? Is it something I should be worried about?

Did you find your problem? It also sounds to me like it could be the rear exhaust manifold. I have a 96 Lumina and have replaced 2 rear manifolds on my 3.1 and about 1 month ago my 2nd replacement exhaust manifold cracked again but I have yet to replace it because I am a little unhappy about this matter. Neither replacent manifolds made it longer than 9 months and my original one lasted 12 years. I am not very happy and I really think the metal going into these replacement manifolds is poor quality, so I have filed a complaint with Oreilly's and will see how it turns out.

Makes sure the engine pipe or the entire exhaust system is not putting pressure on the manifold connection, a major misaligment can cause the manifold to crack, unless it is near the ports.