Noun clause

A noun clause can be used like a noun. It can be a subject, predicate nominative, direct object, appositive, indirect object, or object of the preposition. Some of the words that introduce noun clauses are that, whether, who, why, whom, what, how, when, whoever, where, and whomever. Notice that some of these words also introduce adjective and adverbial clauses. To check whether a clause is a noun clause, try substituting the appropriate pronoun (he, she, it, or they). A noun clause is a dependent clause and cannot stand alone as a sentence.  It must be connected to an independent clause, a main clause. A noun clause has its own subject and verb. It can begin with a question word. It can begin with if or whether. And it can begin with that. 
Examples:

• I know who said that. (I know it.)
• Whoever said it is wrong. (He/she is wrong.)

Sometimes a noun clause is used without the introductory word.
Example:

• I know that he is here.
• I know he is here. (without "that")

In some cases, use of the introductory word, though grammatically correct, may sound cumbersome in English.
Example:

• I think that it is pretty. (correct, though excessive)
• I think it is pretty. (standard usage)

1. Noun Clauses that Start with a Question

Noun clauses that start with a question are usually used to answer a question.  The following examples should better explain this.

• Where does Sarik live?
I don't know where Sarik lives. "where Sarik lives" is a noun clause.  We could answer this question without a noun clause by saying the following. I don't know Sarik's address.  The noun phrase, Sarik's address, replaces with the noun clause, where Sarik lives.    
• What time is it?
I don't know what time it is. "what time it is" is a noun clause.  We could answer this question without a noun clause by saying the following. I don't know the time.  In this case, the noun phrase, the time, replaces the noun clause, what time it is.

2.  Noun Clauses that Start with Whether or If

Noun clauses that start with whether or if are used to answer yes/no type questions.  Whether and if are usually interchangeable.  The following examples should better explain this.

• Does Judy own a Honda?
I don't know if Judy owns a Honda. "if Judy owns a Honda" is a noun clause.  We could answer this question without a noun clause by saying the following. I don't know the answer.  In this case, the noun phrase, the answer, replaces the noun clause, if Judy owns a Honda.   
• Will Sadine be at work on Friday?
I don't know whether Sadine will be at work on Friday. "whether Sadine will be at work on Friday" is a noun clause.  We could answer this question without a noun clause by saying the following. I don't know the answer.  In this case, the noun phrase, the answer, replaces the noun clause, whether Sadine will be at work on Friday.

3.  Noun Clauses that Start with That.

Noun clauses that start with that are used to answer questions in which person who is answering is thinking, giving an opinion, or using a mental activity verb.  The following examples should better explain this.

• Is Dr. Elimelech a good instructor?
I think that Dr. Elimelech is a good instructor. "that Dr. Elimelech is a good instructor" is a noun clause.  This noun clause could be omitted by saying the following. I think so.  
• Do you know the location of an ATM?
I believe that there is an ATM in the supermarket. "that there is an ATM in the supermarket" is a noun clause.

Most of the time, native speakers will drop the word that.  It is perfectably acceptable to say the following. I think that Dr. Elimelech is a good instructor.
OR  I think Dr. Elimelech is a good instructor.
I believe that there is an ATM in the supermarket.
OR  I believe there is an ATM in the supermarket.

 

Source:
http://en.wikipedia.org/wiki/Noun_clause#Noun_clause
http://esl.lbcc.cc.ca.us/eesllessons/nounclauses/nounclause.htm