Jimmy sounds like a diesel when cold

I just bought a 95 Jimmy SLT in amazing condition but I hear a sound from the engine when its cold (only first thing in the morning) when I put a small load on the engine you hear it best...sounds just like a damn diesel...anyone else have or heard of this problem? I have papers and a receipt that the engine was changed 1 year ago for over $4000 at GM
I will have to check and see if there is still a guarantee on that engine but for know I am a little worried. Also I will be doing an oil change this weekend should I put 5W-30 or 10W-30 please remember folks I live in Montreal Canada where it gets pretty cold in the winter here. I also bought a bottle of DuraLube which I will put in.

Thanks

Well I changed the oil and put a new filter and still the same noise when it's cold...which is now almost al of the time :(
I used 5W30 oil but like I said still the same results.

No one has any comments or suggestions that may possibly help?

I also have a 95 jimmy bought last year. This fall the fan clutch went out. I had a new fan clutch installed. After installation I noticed when the vehicle was cold the was a loud humming noise for the first minute or two the engine was running and then is quieted down. I asked the mechanic if this was right and he told me this fan clutch was something like a high reciprocating which was called for in my model of jimmy and is supposed to sound like that. I would guess this is the same sound you are hearing.

I noticed your jimmy was an slt that is the same model I have, it also has the stiffer shocks and skid plate. Maybe this model has something to do with the different kind of fan clutch installed which is making the noise. About the oil to use, I live in North Dakota which has some of the coldest winters around. I would definately run 5w-30 at least in the winter if it gets cold there.

Thanks for your reply Vikings...but did this sound you are talking about sound like a diesel truck? If so then I guess it could be the same but mine is more then 2mins...its more like 20 mins!

It does sound like a diesel for a few minutes when cold but the quiets down, there is still a little hum but not bad. Another thing you may want to look at it the fan shroud. When I bought mine the fan was rubbing on the shroud which also makes a different noise. I have talked with other people who also have had to modify or space out there shroud a bit more.

Your problem sounds like a common case of PISTON SLAP. I have the same problem when it is cold with my 99 blazer. The dealer told me that they can't do much about it if this only happens when it is cold. They say that it doesn't hurt the engine ( I tend to dissagree). If this happens when your engine is at running temp. and is constant this could cost big money to get pistons replaced or replace the engine. EEEPP!